CN110291678B - Secondary battery and method for using secondary battery - Google Patents

Secondary battery and method for using secondary battery Download PDF

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CN110291678B
CN110291678B CN201880011456.4A CN201880011456A CN110291678B CN 110291678 B CN110291678 B CN 110291678B CN 201880011456 A CN201880011456 A CN 201880011456A CN 110291678 B CN110291678 B CN 110291678B
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positive electrode
negative electrode
potential
immediately before
secondary battery
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CN110291678A (en
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加藤有光
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Vision Aesc Japan Co ltd
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Envision Aesc Energy Components Co ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
    • H01M10/44—Methods for charging or discharging
    • H01M10/446—Initial charging measures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
    • H01M10/44—Methods for charging or discharging
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
    • H01M10/44—Methods for charging or discharging
    • H01M10/448—End of discharge regulating measures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/133—Electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H01M4/1391—Processes of manufacture of electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/483—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides for non-aqueous cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
    • H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/60—Selection of substances as active materials, active masses, active liquids of organic compounds
    • H01M4/602—Polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
    • H01M2010/4292—Aspects relating to capacity ratio of electrodes/electrolyte or anode/cathode
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

A secondary battery (10) of the present invention is provided with at least: a positive electrode (11); a negative electrode (12); a separation layer (5) that spatially separates the positive electrode (11) and the negative electrode (12); and an ion conductor held between the positive electrode (11) and the negative electrode (12) and having a function of conducting ions between the positive electrode (11) and the negative electrode (12). The secondary battery (10) has the following characteristics in the initial stage of use: a characteristic that the rate of decrease in the potential of the positive electrode (11) immediately before the end of full discharge is greater than the rate of increase in the potential of the negative electrode (12) immediately before the end of full discharge; and a characteristic that the rate of increase in the potential of the positive electrode (11) immediately before the end of full charge is greater than the rate of decrease in the potential of the negative electrode (12) immediately before the end of full charge, the secondary battery (10) is continuously used until the rate of decrease in the potential of the positive electrode (11) immediately before the end of full discharge is less than the rate of increase in the potential of the negative electrode (12) immediately before the end of full discharge.

Description

Secondary battery and method for using secondary battery
Technical Field
The present invention relates to a secondary battery and a method of using the secondary battery.
Background
As a technique related to the increase in the life of a secondary battery such as a lithium ion secondary battery, for example, a technique described in patent document 1 is cited.
Patent document 1 describes a method for charging and discharging a lithium secondary battery including: the lithium ion secondary battery is characterized by comprising a positive electrode having a positive electrode active material capable of occluding/releasing lithium ions, a negative electrode having a negative electrode active material capable of occluding/releasing lithium ions, a separator arranged between the positive electrode and the negative electrode, and an electrolyte having lithium ion conductivity, wherein the positive electrode active material contains a lithium-containing transition metal oxide, the negative electrode has a reversible capacity greater than the utilization capacity of the positive electrode, and first charge/discharge is performed in which the positive electrode after charging is discharged to a first potential VDp1 of less than 2.7V relative to lithium metal, and the discharge is terminated.
Documents of the prior art
Patent document
Patent document 1: japanese patent No. 5117638
Disclosure of Invention
Problems to be solved by the invention
In the method of patent document 1, it is necessary to control the positive electrode potential at the time of discharge to a value of less than 2.7V (Li +/Li). Since the positive electrode and the negative electrode are deteriorated depending on the usage environment, the relationship between the positive electrode potential and the negative electrode potential changes with the deterioration. Thus, there are the following problems: the positive electrode potential cannot be estimated from the potential between the positive electrode and the negative electrode, and a procedure for accurately evaluating the positive electrode potential is required. Further, it can be considered that: if the potential of the negative electrode is increased to about 2.7V (Li +/Li) before the positive electrode potential becomes less than 2.7V (Li +/Li) during discharge due to degradation and the potential between the positive electrode and the negative electrode approaches zero volts, the positive electrode potential may not be made less than 2.7V (Li +/Li). There are also problems here: even if the capacity that can be operated remains in both the positive electrode and the negative electrode, the battery cannot be used.
The present invention has been made in view of the above circumstances, and provides a secondary battery having a long product life and a method of using a secondary battery capable of extending the product life.
Means for solving the problems
The present inventors have made extensive studies to achieve the above object. As a result, they found that: the present inventors have found that the product life of a secondary battery can be prolonged by setting the charge/discharge characteristics of a positive electrode and a negative electrode at the initial stage of use of the secondary battery to a specific relationship and continuing the use of the secondary battery until the rate of decrease in the potential of the positive electrode immediately before the end of full discharge becomes smaller than the rate of increase in the potential of the negative electrode immediately before the end of full discharge, and have completed the present invention.
That is, according to the present invention, there is provided a secondary battery including at least:
a positive electrode;
a negative electrode;
a separation layer spatially separating the positive electrode and the negative electrode; and
an ion conductor held between the positive electrode and the negative electrode and having a function of conducting ions between the positive electrode and the negative electrode,
the secondary battery has the following characteristics in the initial stage of use:
a characteristic in which a rate of decrease in the potential of the positive electrode immediately before completion of full discharge is greater than a rate of increase in the potential of the negative electrode immediately before completion of full discharge; and
a characteristic that a rate of increase in the potential of the positive electrode immediately before completion of full charge is larger than a rate of decrease in the potential of the negative electrode immediately before completion of full charge,
the secondary battery is continuously used until the rate of decrease in the potential of the positive electrode immediately before the end of full discharge is smaller than the rate of increase in the potential of the negative electrode immediately before the end of full discharge.
Further, according to the present invention, there is provided a method of using a secondary battery, the method including at least:
a positive electrode;
a negative electrode;
a separation layer spatially separating the positive electrode and the negative electrode; and
an ion conductor held between the positive electrode and the negative electrode and having a function of conducting ions between the positive electrode and the negative electrode,
at the initial stage of use of the secondary battery,
the positive electrode is used under the condition that the potential decrease rate of the positive electrode immediately before the end of full discharge is larger than the potential increase rate of the negative electrode immediately before the end of full discharge, and the positive electrode is used under the condition that the potential increase rate of the positive electrode immediately before the end of full charge is larger than the potential decrease rate of the negative electrode immediately before the end of full charge,
the secondary battery is continuously used until the rate of decrease in the potential of the positive electrode immediately before the end of full discharge is smaller than the rate of increase in the potential of the negative electrode immediately before the end of full discharge.
Effects of the invention
According to the present invention, a secondary battery having a long product life and a method of using a secondary battery capable of extending the product life can be provided.
Drawings
The above objects and other objects, features and advantages will be further apparent from the following description of suitable embodiments and the accompanying drawings described below.
Fig. 1 is a sectional view showing an example of the structure of a secondary battery according to an embodiment of the present invention.
Fig. 2 is a diagram showing an example of the relationship between the charge/discharge characteristics of the positive electrode and the charge/discharge characteristics of the negative electrode in the secondary battery according to the embodiment of the present invention, fig. 2 (a) shows an example of the relationship between the charge/discharge characteristics of the positive electrode and the charge/discharge characteristics of the negative electrode in the initial stage of use of the secondary battery, and fig. 2 (b) shows an example of the relationship between the charge/discharge characteristics of the positive electrode and the charge/discharge characteristics of the negative electrode in the later stage of use of the secondary battery.
Fig. 3 is a diagram showing an example of the relationship between the initial charge/discharge characteristics of the positive electrode and the initial charge/discharge characteristics of the negative electrode when an electrode element having a larger irreversible capacity of the positive electrode than that of the negative electrode is used.
Fig. 4 is a schematic diagram for explaining the correction of the relationship between the first charge-discharge characteristics of the positive electrode and the first charge-discharge characteristics of the negative electrode in example 1.
Fig. 5 is a schematic diagram for explaining the correction of the relationship between the first charge-discharge characteristics of the positive electrode and the first charge-discharge characteristics of the negative electrode in example 2.
Fig. 6 is a diagram showing an example of the relationship between the initial charge-discharge characteristic of the positive electrode and the initial charge-discharge characteristic of the negative electrode when an electrode element having a larger irreversible capacity of the negative electrode than the positive electrode is used.
Fig. 7 is a schematic diagram for explaining the correction of the relationship between the first charge-discharge characteristics of the positive electrode and the first charge-discharge characteristics of the negative electrode in example 3.
Fig. 8 is a schematic diagram for explaining the correction of the relationship between the first charge-discharge characteristics of the positive electrode and the first charge-discharge characteristics of the negative electrode in example 4.
Detailed Description
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, the same components are denoted by the same reference numerals, and the description thereof is omitted. In the drawings, the respective constituent elements schematically show shapes, sizes, and arrangement relationships to the extent that the present invention can be understood, which are different from actual sizes. In the present embodiment, "a to B" in the numerical range represent a to B, unless otherwise specified.
< Secondary Battery and method of Using Secondary Battery >
The secondary battery 10 and the method of using the secondary battery 10 according to the present embodiment will be described below. Fig. 1 is a sectional view showing an example of the structure of a secondary battery 10 according to an embodiment of the present invention. Fig. 2 is a diagram showing an example of the relationship between the charge/discharge characteristics of the positive electrode 11 and the charge/discharge characteristics of the negative electrode 12 in the secondary battery 10 according to the embodiment of the present invention, (a) shows an example of the relationship between the charge/discharge characteristics of the positive electrode 11 and the charge/discharge characteristics of the negative electrode 12 in the initial stage of use of the secondary battery 10, and (b) shows an example of the relationship between the charge/discharge characteristics of the positive electrode 11 and the charge/discharge characteristics of the negative electrode 12 in the later stage of use of the secondary battery 10.
As shown in fig. 1, the secondary battery 10 according to the present embodiment includes at least: a positive electrode 11; a negative electrode 12; a separation layer 5 for spatially separating the positive electrode 11 and the negative electrode 12; and an ion conductor held between the positive electrode 11 and the negative electrode 12 and having a function of conducting ions between the positive electrode 11 and the negative electrode 12. The secondary battery 10 has the following characteristics in an initial stage of use: a characteristic in which the rate of decrease in the potential of the positive electrode 11 immediately before the end of full discharge is greater than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge; and a characteristic that the rate of increase in the potential of the positive electrode 11 immediately before the end of full charge is greater than the rate of decrease in the potential of the negative electrode 12 immediately before the end of full charge, the secondary battery 10 is continuously used until the rate of decrease in the potential of the positive electrode 11 immediately before the end of full discharge is less than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge.
That is, in the initial stage of use of the secondary battery 10, the secondary battery 10 is used under the condition that the rate of decrease in the potential of the positive electrode 11 immediately before the end of full discharge is larger than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge, and under the condition that the rate of increase in the potential of the positive electrode 11 immediately before the end of full charge is larger than the rate of decrease in the potential of the negative electrode 12 immediately before the end of full charge, and is continued to be used until the rate of decrease in the potential of the positive electrode 11 immediately before the end of full discharge is smaller than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge.
Here, in the present embodiment, the term "full-discharge end" refers to a state in which the battery voltage reaches a voltage set as a discharge end voltage, and the term "full-charge end" refers to a state in which the battery voltage reaches a voltage set as a charge end voltage. The discharge end voltage and the charge end voltage are set according to the combination of the positive electrode and the negative electrode, and can be determined based on known information.
In the present embodiment, the later stage of use means: during the use period of the secondary battery 10, a part of conductive ions contributing to charge and discharge is mixed into a reaction product obtained by decomposition of the electrolyte and the like and is reduced, and a portion of the negative electrode 12 used in the capacity is displaced, and a state is assumed in which the rate of decrease in the potential of the positive electrode 11 immediately before the end of full discharge is smaller than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge.
The positive electrode 11 has: a positive electrode current collector 3 made of a metal foil such as an aluminum foil, and a positive electrode active material layer 1 provided on one surface of the positive electrode current collector 3 and containing a positive electrode active material.
The negative electrode 12 has: a negative electrode current collector 4 made of a metal foil such as a copper foil, and a negative electrode active material layer 2 provided on one surface of the negative electrode current collector 4 and containing a negative electrode active material.
The positive electrode 11 and the negative electrode 12 are stacked with the separator 5 interposed therebetween such that the positive electrode active material layer 1 and the negative electrode active material layer 2 face each other. For the separation layer 5, for example, a separator made of a nonwoven fabric, a microporous film made of polyolefin (polypropylene, polyethylene, or the like), or the like can be used.
The electrode elements including the positive electrode 11, the separator 5, and the negative electrode 12 are housed in a container including the outer packages 6 and 7, for example. The exterior bodies 6 and 7 may use, for example, an aluminum laminated film.
The positive electrode current collector 3 is connected to a positive electrode tab 9, and the negative electrode current collector 4 is connected to a negative electrode tab 8, which are led out of the container. Injecting the ion conductor into the container and sealing. An electrode group in which a plurality of electrode elements are stacked may be housed in the container.
Next, the relationship between the charge and discharge characteristics of the positive electrode 11 and the negative electrode 12 of the secondary battery 10 according to the present embodiment will be described.
First, as shown in fig. 2 (a), in the secondary battery 10 according to the present embodiment, the potential of the positive electrode 11 tends to decrease and the potential of the negative electrode 12 tends to increase immediately before the end of full discharge in the initial stage of use of the secondary battery 10.
As shown in fig. 2 (a), the secondary battery 10 according to the present embodiment has a characteristic in which the rate of decrease in the potential of the positive electrode 11 immediately before the end of full discharge is greater than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge in the initial stage of use of the secondary battery 10. As shown in fig. 2 (b), the secondary battery 10 according to the present embodiment is used until the rate of decrease in the potential of the positive electrode 11 immediately before the end of full discharge is smaller than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge.
That is, the secondary battery 10 according to the present embodiment is configured such that the rate of decrease in the potential of the positive electrode 11 per unit capacity or per unit time immediately before the end of full discharge becomes larger in absolute value than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge in the initial stage of use of the secondary battery 10, and the secondary battery 10 is used until the rate of decrease in the potential of the positive electrode 11 immediately before the end of full discharge becomes smaller than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge.
In other words, the end of discharge is mainly determined by the decrease in the potential of the positive electrode at the initial stage of use of the secondary battery 10, and is mainly determined by the increase in the potential of the negative electrode at the later stage of use of the secondary battery 10.
In the secondary battery 10 according to the present embodiment, as shown in fig. 2 (a), the potential of the positive electrode 11 tends to increase and the potential of the negative electrode 12 tends to decrease immediately before the full charge is completed in the initial stage of use of the secondary battery 10.
As shown in fig. 2 (a), the secondary battery 10 according to the present embodiment has a characteristic in which the rate of increase in the potential of the positive electrode 11 immediately before the end of full charge is greater than the rate of decrease in the potential of the negative electrode 12 immediately before the end of full charge, in the initial stage of use of the secondary battery 10. That is, as shown in fig. 2 (a), the secondary battery 10 according to the present embodiment is used under the condition that the rate of increase in the potential of the positive electrode 11 immediately before the end of full charge is larger than the rate of decrease in the potential of the negative electrode 12 immediately before the end of full charge at the initial stage of use of the secondary battery 10.
The secondary battery 10 according to the present embodiment is configured such that the rate of increase in the potential per unit capacity or unit time of the positive electrode 11 immediately before the end of full charge in the initial stage of use of the secondary battery 10 is larger in absolute value than the rate of decrease in the potential of the negative electrode 12 immediately before the end of full charge.
In other words, in the initial stage of use of the secondary battery 10, the end of charging is mainly determined by the increase in the potential of the positive electrode 11.
The secondary battery 10 according to the present embodiment is preferably designed as follows: in the initial stage of use, the absolute value (Δ V) of the amount of change in potential per 10mAh/g of positive electrode 11 immediately before completion of full discharge when discharge is performed at a constant current of 1/20C 2 ) Absolute value (Δ V) of the amount of change in potential per 10mAh/g of the negative electrode 12 immediately before completion of full discharge 1 ) Ratio of (Δ V) 2 /ΔV 1 ) Satisfies Δ V 2 /ΔV 1 A relation of > 1, more preferably designed to satisfy Δ V 2 /ΔV 1 A relationship of > 2.That is, the secondary battery 10 according to the present embodiment is preferably used under the condition that the following relationship is satisfied: in the initial stage of use, the absolute value (Δ V) of the amount of change in potential per 10mAh/g of positive electrode 11 immediately before completion of full discharge when discharging at a constant current of 1/20C 2 ) Absolute value (Δ V) of the amount of change in potential per 10mAh/g of the negative electrode 12 immediately before completion of full discharge 1 ) Ratio of (Δ V) 2 /ΔV 1 ) Satisfies Δ V 2 /ΔV 1 A relation of > 1, more preferably Δ V 2 /ΔV 1 The relationship > 2.
In addition, the secondary battery 10 according to the present embodiment is preferably designed as follows: in the initial stage of use, the absolute value (Δ V) of the amount of change in potential per 10mAh/g of the positive electrode 11 immediately before the end of full charge when charging is performed at a constant current of 1/20C 4 ) Absolute value (Δ V) of the amount of change in potential per 10mAh/g of the negative electrode 12 immediately before completion of full charge 3 ) Ratio of (Δ V) 4 /ΔV 3 ) Satisfies Δ V 4 /ΔV 3 A relation of > 1, more preferably designed to satisfy Δ V 4 /ΔV 3 A relationship of > 2. That is, the secondary battery 10 according to the present embodiment is preferably used under the condition that the following relationship is satisfied: in the initial stage of use, the absolute value (Δ V) of the amount of change in potential per 10mAh/g of the positive electrode 11 immediately before the end of full charge when charging is performed at a constant current of 1/20C 4 ) Absolute value (Δ V) of the amount of change in potential per 10mAh/g of the negative electrode 12 immediately before completion of full charge 3 ) Ratio of (Δ V) 4 /ΔV 3 ) Satisfies Δ V 4 /ΔV 3 A relation of > 1, more preferably Δ V 4 /ΔV 3 The relationship > 2.
In addition, in the secondary battery 10 according to the present embodiment, it is preferable that the secondary battery is continuously used until the absolute value of the amount of change in potential per 10mAh/g of the positive electrode 11 (Δ V) immediately before the completion of full discharge when the battery is discharged at a constant current of 1/20C 2 ) Absolute value (Δ V) of the amount of change in potential per 10mAh/g of the negative electrode 12 immediately before completion of full discharge 1 ) Ratio of (Δ V) 2 /ΔV 1 ) Satisfies Δ V 2 /ΔV 1 Until the state of the relationship of < 1, it is more preferable to continue using the secondary battery until Δ V is satisfied 2 /ΔV 1 A state of relation < 0.5.
According to the secondary battery 10 of the present embodiment, by setting the charge/discharge characteristics of the positive electrode 11 and the negative electrode 12 to satisfy the relationship shown in fig. 2 (a) in the initial stage of use, excess conductive ions can be made to exist in the negative electrode 12 at the end of full discharge in the initial stage of use of the secondary battery 10. Thus, the secondary battery 10 according to the present embodiment can be operated using the entire capacity of the positive electrode 11 and a part of the capacity of the negative electrode 12.
Therefore, even if the capacity of the negative electrode 12 is reduced, the reduction in the capacity of the secondary battery 10 can be suppressed. Further, when a part of the conductive ions contributing to charge and discharge is mixed into a reaction product or the like obtained by decomposition of the electrolytic solution and reduced during the use period of the secondary battery 10, a part of the used part in the capacity of the negative electrode 12 is displaced, and thereby the reduced conductive ions can be replenished with the remaining conductive ions in the negative electrode 12. Therefore, even if a part of the conductive ions contributing to charge and discharge is reduced, a decrease in the capacity of the secondary battery 10 can be suppressed.
As shown in fig. 2 (b), the secondary battery 10 according to the present embodiment can be operated as a battery even when used in a state in which the rate of decrease in the potential of the positive electrode 11 immediately before the end of full discharge is smaller than the rate of increase in the potential of the negative electrode 12 immediately before the end of full discharge. Therefore, according to the secondary battery 10 of the present embodiment, the product life can be extended.
Examples of a method for realizing a secondary battery in which the charge/discharge characteristics of the positive electrode 11 and the negative electrode 12 satisfy the relationship shown in fig. 2 (a) and (b) include the following: a method of using a positive electrode 11 obtained by performing a treatment for removing a part of conductive ions in the positive electrode 11, a method of using a negative electrode 12 including a material having an irreversible capacity, a method of using a negative electrode 12 to which conductive ions are added, a method of using a positive electrode 11 obtained by performing a chemical treatment for removing or adding a part of conductive ions in the positive electrode 11, and the like.
Examples of the material having irreversible capacity include polyimide, silicon, and the like. The treatment of adding conductive ions to the positive electrode 11 may be an over-discharge treatment.
More specifically, the following methods 1 to 4 can be mentioned as a method for realizing a secondary battery in which the charge-discharge characteristics of the positive electrode 11 and the negative electrode 12 satisfy the relationship shown in fig. 2 (a) and (b).
(method 1) method in which a positive electrode having a larger irreversible capacity than that of the negative electrode is subjected to a treatment (e.g., chemical treatment) for removing a part of conductive ions (see example 1 described later)
(method 2) method of using a negative electrode comprising a negative electrode active material having an irreversible capacity smaller than that of the positive electrode and a material having an irreversible capacity (for example, polyimide or silicon) (see example 2 described later)
(method 3) A method in which a negative electrode having a larger irreversible capacity than the positive electrode is subjected to a treatment (e.g., chemical treatment) for adding a conductive ion thereto is used (see example 3 described later)
(method 4) A method in which a positive electrode having a smaller irreversible capacity than that of the negative electrode is subjected to a treatment (e.g., chemical treatment or overdischarge treatment) for adding a conductive ion thereto was used (see example 4 described later)
A plurality of secondary batteries 10 according to the present embodiment may be combined to form a battery pack. The secondary battery 10 or the battery pack thereof according to the present embodiment can be suitably used for applications such as an electric storage system and an automobile battery.
The secondary battery 10 according to the present embodiment is, for example, a lithium-ion secondary battery.
Next, each component constituting the secondary battery 10 according to the present embodiment will be described.
(Positive electrode)
The positive electrode 11 constituting the secondary battery 10 according to the present embodiment includes: a positive electrode current collector 3 made of a metal foil such as an aluminum foil, and a positive electrode active material layer 1 provided on one surface of the positive electrode current collector 3 and containing a positive electrode active material.
The positive electrode active material is not particularly limited as long as it contains a material capable of storing and releasing lithium. For example, LiMn may be used 2 O 4 Or LiCoO 2 And 4V class (average working potential is 3.6-3.8V: relative to lithium potential) materials are obtained. These positive electrode active materials undergo a redox reaction (Co) of Co ions or Mn ions 3+ ←→Co 4+ Or Mn 3+ ←→Mn 4+ ) To define the presentation potential.
Further, as the positive electrode active material, LiMlO may also be used 2 (M1 is at least 1 element selected from Mn, Fe, Co and Ni, and part of M1 is optionally substituted with Mg, Al or Ti), LiMn 2-x M2 x O 4 (M2 is at least one element selected from Mg, Al, Co, Ni, Fe and B, x is 0-0.4) and the like; LiFePO 4 The olivine-type material shown, and the like.
From the viewpoint of obtaining a high energy density, it is preferable to include a positive electrode active material capable of occluding or releasing lithium ions at a potential of 4.5V or more with respect to lithium metal.
The positive electrode active material having a potential of 4.5V or more with respect to lithium metal can be selected by, for example, the following method. First, a positive electrode containing a positive electrode active material and Li metal were placed in a container in a state of facing each other with a separator interposed therebetween, and then an electrolyte solution was injected into the container to produce a battery. When the positive electrode active material in the positive electrode is charged and discharged at a constant current of, for example, 5mAh/g, the positive electrode active material can be a material that has a charge/discharge capacity of 10mAh/g or more per unit mass of the positive electrode active material at a potential of 4.5V or more per lithium metal, and that operates at a potential of 4.5V or more per lithium metal.
For example, it is known that: by using a spinel compound in which Mn of lithium manganate is substituted with Ni, Co, Fe, Cu, Cr, or the like, as a positive electrode active material, an operating potential of 5V class can be realized. Specifically, LiNi is known 0.5 Mn 1.5 O 4 The spinel compound exhibits a plateau in the region of 4.5V or more. In such spinel compoundsIn which Mn is present in a state of valence 4 and Ni is used 2+ ←→Ni 4+ Redox replacement of Mn 3+ ←→Mn 4+ The redox of (a) dictates the operating potential.
For example, LiNi 0.5 Mn 1.5 O 4 Has a capacity of 130mAh/g or more and an average operating voltage of 4.6V or more with respect to lithium metal. Capacity ratio of LiCoO 2 Small, but energy density of the battery is in comparison with LiCoO 2 High. Further, the spinel-type lithium manganese oxide has advantages such as having a three-dimensional lithium diffusion path, excellent thermodynamic stability, and easy synthesis.
As a positive electrode active material that operates at a potential of 4.5V or more with respect to lithium metal, for example, there is a lithium manganese composite oxide represented by the following formula (1). The lithium manganese composite oxide represented by the following formula (1) is a positive electrode active material that operates at a potential of 4.5V or more with respect to lithium metal.
Li a (M x Mn 2-x-y Y y )(O 4-w Z w ) (1)
(wherein x is 0.3. ltoreq. x.ltoreq.1.2, Y is 0. ltoreq. Y, x + Y < 2, a is 0. ltoreq. a.ltoreq.1.2, w is 0. ltoreq. w.ltoreq.1. M is at least one member selected from the group consisting of Co, Ni, Fe, Cr and Cu, Y is at least one member selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K and Ca, Z is at least one member selected from the group consisting of F and Cl.)
The lithium manganese complex oxide represented by the above formula (1) is more preferably a compound represented by the following formula (1-1).
Li a (M x Mn 2-x-y Y y )(O 4-w Z w ) (1-1)
(wherein x is 0.5. ltoreq. x.ltoreq.1.2, Y is 0. ltoreq. Y, x + Y < 2, a is 0. ltoreq. a.ltoreq.1.2, w is 0. ltoreq. w.ltoreq.1. M is at least one member selected from the group consisting of Co, Ni, Fe, Cr and Cu, Y is at least one member selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K and Ca, Z is at least one member selected from the group consisting of F and Cl.)
In the formula (1), M preferably contains Ni, and more preferably is only Ni. This is because: when M contains Ni, a high-capacity positive electrode active material can be obtained relatively easily. In the case where M consists of Ni, fromFrom the viewpoint of a high-capacity positive electrode active material, x is preferably 0.4 or more and 0.6 or less. Further, when the positive electrode active material is LiNi 0.5 Mn 1.5 O 4 It is more preferable because a high capacity of 130mAh/g or more can be obtained.
Further, as the positive electrode active material represented by the above formula (1) that operates at a potential of 4.5V or more with respect to lithium metal, for example, LiCrMnO can be mentioned 4 、LiFeMnO 4 、LiCoMnO 4 、LiCu 0.5 Mn 1.5 O 4 And the capacity of these positive electrode active materials is high. In addition, the positive electrode active material may be prepared by mixing these active materials with LiNi 0.5 Mn 1.5 O 4 And mixing the components.
In addition, by substituting a part of the Mn portion of these active materials with Li, B, Na, Al, Mg, Ti, SiK, Ca, or the like, the lifetime can be improved in some cases. In other words, in the above formula (1), 0 < y may improve the lifetime. Among these, when Y is Al, Mg, Ti or Si, the effect of improving the lifetime is high. Further, when Y is Ti, it is more preferable because the effect of improving the lifetime is exhibited while maintaining a high capacity. The range of y is preferably greater than 0 and 0.3 or less. By setting y to 0.3 or less, the decrease in capacity is easily suppressed.
In addition, the oxygen moiety may be substituted with F, Cl. In the above formula (1), by setting w to be greater than 0 and 1 or less, a decrease in capacity can be suppressed.
Examples of the spinel-type positive electrode active material represented by the above formula (1) include LiNi 0.5 Mn 1.5 O 4 And the like as M and Ni; and LiCr x Mn 2-x O 4 (0.4≤x≤1.1)、LiFe x Mn 2-x O 4 (0.4≤x≤1.1)、LiCu x Mn 2-x O 4 (0.3≤x≤0.6)、LiCo x Mn 2-x O 4 (x is more than or equal to 0.4 and less than or equal to 1.1), and the like; and solid solutions thereof.
Further, as the positive electrode active material that operates at a potential of 4.5V or more with respect to lithium metal, an olivine-type positive electrode can be mentionedA very active material. An example of the olivine-type positive electrode active material is LiMPO 4 (M: at least one of Co and Ni), e.g., LiCoPO 4 Or LiNiPO 4 And so on.
Further, as the positive electrode active material that operates at a potential of 4.5V or more with respect to lithium metal, there may be also mentioned a Si composite oxide, and as the Si composite oxide, for example, Li is mentioned 2 MSiO 4 (M is at least one of Mn, Fe and Co).
The positive electrode active material that operates at a potential of 4.5V or more with respect to lithium metal also includes a material having a layered structure, and examples of the positive electrode active material including a layered structure include Li (M1) x M2 y Mn 2-x-y )O 2 (M1: at least one member selected from the group consisting of Ni, Co and Fe; M2: at least one member selected from the group consisting of Li, Mg and Al; 0.1 < x < 0.5, 0.05 < y < 0.3), Li (M 1-z Mn z )O 2 (M is at least one of Li, Co and Ni, and 0.7. gtoreq.z.gtoreq.0.33) or Li (Li) x M 1-x-z Mn z )O 2 (M is at least one of Co and Ni, 0.3. gtoreq.0.1, 0.7. gtoreq.z. gtoreq.0.33).
The specific surface area of the positive electrode active material such as the lithium manganese composite oxide represented by the above formula (1) is, for example, 0.01 to 5m 2 A ratio of 0.05 to 4 m/g 2 A concentration of 0.1 to 3m 2 A more preferable range is 0.2 to 2m 2 (ii) in terms of/g. By setting the specific surface area in such a range, the contact area with the electrolyte can be adjusted to an appropriate range. In other words, by setting the specific surface area to be equal to or greater than the lower limit value, the lithium ions can be easily and smoothly inserted and desorbed, and the resistance can be further reduced. Further, by setting the specific surface area to the upper limit or less, acceleration of decomposition of the electrolytic solution and elution of the constituent elements of the active material can be further suppressed.
The center particle diameter (median diameter: D) of the active material such as the above-mentioned lithium manganese composite oxide 50 ) Preferably 0.1 to 50 μm, more preferably 0.2 to 40 μm. By setting the particle diameter to be not less than the lower limit, the constituent elements such as Mn can be further suppressedThe elution of the element can further suppress the deterioration due to the contact with the electrolyte. Further, by setting the particle diameter to the upper limit or less, it is possible to easily and smoothly carry out the intercalation and deintercalation of lithium ions, and further reduce the resistance.
The particle size can be measured by a laser diffraction/scattering particle size distribution measuring apparatus.
The positive electrode active material may be used alone in 1 kind, or in combination with 2 or more kinds.
For example, the positive electrode active material may be contained only in the 4V class. From the viewpoint of obtaining a high energy density, it is more preferable to use a positive electrode active material that operates at a potential of 4.5V or more with respect to lithium metal as described above. Further, a 4V-grade positive electrode active material may be contained.
The binder for the positive electrode is not particularly limited, and examples thereof include polyvinylidene fluoride (PVDF), a vinylidene fluoride-hexafluoropropylene copolymer, a vinylidene fluoride-tetrafluoroethylene copolymer, a styrene-butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, and polyamideimide. Among these, polyvinylidene fluoride is preferable from the viewpoint of versatility and low cost.
From the viewpoint of "sufficient binding power" and "high energy" in this trade-off relationship, the amount of the binder for a positive electrode used is preferably 2 to 10 parts by mass per 100 parts by mass of the positive electrode active material.
The positive electrode active material layer 1 containing the positive electrode active material may be added with a conductive assistant for the purpose of reducing resistance. Examples of the conductive assistant include carbonaceous fine particles such as graphite, carbon black, and acetylene black.
The positive electrode current collector 3 is not particularly limited, and is preferably composed of one or two or more selected from aluminum, nickel, copper, silver, and alloys thereof, and stainless steel, from the viewpoint of electrochemical stability. Examples of the shape of the positive electrode current collector 3 include a foil, a flat plate, and a mesh.
The positive electrode 11 can be produced by, for example, forming a positive electrode active material layer 1 containing a positive electrode active material and a positive electrode binder on a positive electrode current collector 3. Examples of the method for forming the positive electrode active material layer 1 include a doctor blade method, a die coating method, a CVD method, a sputtering method, and the like. The positive electrode current collector 3 may be prepared by forming the positive electrode active material layer 1 in advance, and then forming a thin film of aluminum, nickel, or an alloy thereof by a method such as vapor deposition or sputtering.
(cathode)
The negative electrode 12 constituting the secondary battery 10 according to the present embodiment includes: a negative electrode current collector 4 made of a metal foil such as a copper foil; and a negative electrode active material layer 2 provided on one surface of the negative electrode current collector 4 and containing a negative electrode active material.
The negative electrode active material is not particularly limited as long as it contains a material capable of occluding and releasing lithium, and examples thereof include a carbon material (a) capable of occluding and releasing lithium ions, a metal (b) capable of forming an alloy with lithium, and a metal oxide (c) capable of occluding and releasing lithium ions.
As the carbon material (a), graphite (natural graphite, artificial graphite, or the like), amorphous carbon, diamond-like carbon, carbon nanotubes, or a composite thereof, or the like can be used.
Here, graphite having high crystallinity has high conductivity, and is excellent in adhesion to the negative electrode current collector 4 containing a metal such as copper and in voltage flatness. On the other hand, since amorphous carbon having low crystallinity has small volume expansion, the effect of relaxing the volume expansion of the entire negative electrode is high, and deterioration due to unevenness such as grain boundaries and defects is less likely to occur.
The carbon material (a) may be used alone or in combination with other substances. In an embodiment used in combination with another substance, the carbon material (a) is preferably in a range of 2% by mass or more and 80% by mass or less, and more preferably in a range of 2% by mass or more and 30% by mass or less in the negative electrode active material.
Examples of the metal (b) include metals mainly composed of Al, Si, Pb, Sn, Zn, Cd, Sb, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, La, and the like; an alloy of 2 or more of these metals; alloys of these metals or alloys with lithium, and the like. The metal (b) is more preferably silicon (Si).
The metal (b) may be used alone or in combination with other substances. In an embodiment used in combination with another substance, the metal (b) is preferably in a range of 5% by mass or more and 90% by mass or less, and more preferably in a range of 20% by mass or more and 50% by mass or less in the negative electrode active material.
As the metal oxide (c), for example, silicon oxide, aluminum oxide, tin oxide, indium oxide, zinc oxide, lithium oxide, or a composite thereof can be used. As the metal oxide (c), silicon oxide is more preferably contained. This is because: silica is relatively stable and does not readily react with other compounds.
In addition, for example, 0.1 to 5 mass% of one or two or more elements selected from nitrogen, boron, and sulfur may be added to the metal oxide (c). This can improve the conductivity of the metal oxide (c).
The metal oxide (c) may be used alone or in combination with other substances. In an embodiment used in combination with other materials, the metal oxide (c) is preferably in the range of 5% by mass or more and 90% by mass or less, more preferably in the range of 40% by mass or more and 70% by mass or less in the anode active material.
Examples of the metal oxide (c) include LiFe 2 O 3 、WO 2 、MoO 2 、SiO、SiO 2 、CuO、SnO、SnO 2 、Nb 3 O 5 、Li x Ti 2-x O 4 (1≤x≤4/3)、PbO 2 、Pb 2 O 5 And the like.
In addition to the above, examples of the negative electrode active material include, for example, a metal sulfide (d) capable of occluding and releasing lithium ions. Examples of the metal sulfide (d) include SnS and FeS 2 And the like.
In addition, as the negative electrode active material, for example, metal lithium; a lithium alloy; polyacene; a polythiophene; li 5 (Li 3 N)、Li 7 MnN 4 、Li 3 FeN 2 、Li 2.5 Co 0.5 N、Li 3 Lithium nitride such as CoN.
The negative electrode active material may be used alone or in combination of two or more.
In addition, the negative electrode active material may be configured to include a carbon material (a), a metal (b), and a metal oxide (c). The negative electrode active material will be described below.
The metal oxide (c) preferably has an amorphous structure in whole or in part. The amorphous metal oxide (c) can suppress volume expansion of the carbon material (a) and the metal (b), and can suppress decomposition of the electrolytic solution. The mechanism is presumed to be: since the metal oxide (c) has an amorphous structure, it has some influence on the formation of a coating at the interface between the carbon material (a) and the electrolyte. Further, it is considered that the amorphous structure has fewer elements due to unevenness such as grain boundaries and defects. Note that the fact that all or a part of the metal oxide (c) has an amorphous structure can be confirmed by X-ray diffraction measurement (general XRD measurement). Specifically, in the case where the metal oxide (c) does not have an amorphous structure, a peak unique to the metal oxide (c) is observed, but in the case where all or a part of the metal oxide (c) has an amorphous structure, a peak unique to the metal oxide (c) is observed to be widened.
The metal oxide (c) is preferably an oxide of a metal constituting the metal (b). Further, the metal (b) and the metal oxide (c) are preferably silicon (Si) and silicon oxide (SiO), respectively.
The metal (b) is preferably dispersed in whole or in part in the metal oxide (c). By dispersing at least a part of the metal (b) in the metal oxide (c), the volume expansion of the entire negative electrode can be further suppressed, and the decomposition of the electrolytic solution can be also suppressed. The dispersion of all or a part of the metal (b) in the metal oxide (c) can be confirmed by a combination of transmission Electron microscope observation (general tem (transmission Electron microscope) observation) and Energy Dispersive X-ray spectroscopy measurement (general EDX (Energy Dispersive X-ray spectroscopy) measurement). Specifically, it was confirmed that the metal constituting the metal (b) particles did not form an oxide by observing the cross section of the sample containing the metal (b) particles and measuring the oxygen concentration of the metal (b) particles dispersed in the metal oxide (c).
As described above, the content of each of the carbon material (a), the metal (b), and the metal oxide (c) with respect to the total of the carbon material (a), the metal (b), and the metal oxide (c) is preferably 2 mass% or more and 80 mass% or less, 5 mass% or more and 90 mass% or less, and 5 mass% or more and 90 mass% or less, respectively. The content of each of the carbon material (a), the metal (b), and the metal oxide (c) with respect to the total of the carbon material (a), the metal (b), and the metal oxide (c) is more preferably 2 mass% or more and 30 mass% or less, 20 mass% or more and 50 mass% or less, and 40 mass% or more and 70 mass% or less, respectively.
The negative electrode active material in which all or a part of the metal oxide (c) has an amorphous structure and all or a part of the metal (b) is dispersed in the metal oxide (c) can be produced by, for example, the method disclosed in japanese patent application laid-open No. 2004-47404. That is, by subjecting the metal oxide (c) to CVD treatment in an atmosphere containing an organic gas such as methane gas, a composite in which the metal (b) in the metal oxide (c) is nano-clustered and the surface is covered with the carbon material (a) can be obtained. The negative electrode active material can also be produced by mixing the carbon material (a), the metal (b), and the metal oxide (c) by mechanical grinding.
The carbon material (a), the metal (b), and the metal oxide (c) are not particularly limited, and particulate materials may be used. For example, the average particle diameter of the metal (b) may be smaller than the average particle diameter of the carbon material (a) and the average particle diameter of the metal oxide (c). In this way, the particle size of the metal (b) having a large volume change during charge and discharge is relatively small, and the particle sizes of the carbon material (a) and the metal oxide (c) having a small volume change are relatively large, so that the generation of dendrites and the micronization of the alloy are more effectively suppressed.
In addition, the occurrence of residual stress and residual strain is also suppressed from the viewpoint of storing and releasing lithium in the order of large-particle size particles, small-particle size particles, and large-particle size particles during charge and discharge. The average particle diameter of the metal (b) may be, for example, 20 μm or less, preferably 15 μm or less.
The average particle size of the metal oxide (c) is preferably 1/2 or less of the average particle size of the carbon material (a), and the average particle size of the metal (b) is preferably 1/2 or less of the average particle size of the metal oxide (c). Further, it is more preferable that the average particle diameter of the metal oxide (c) is 1/2 or less of the average particle diameter of the carbon material (a), and the average particle diameter of the metal (b) is 1/2 or less of the average particle diameter of the metal oxide (c). When the average particle diameter is controlled to such a range, the effect of relaxing the volume expansion of the metal and alloy phases can be more effectively obtained, and a secondary battery having an excellent balance among energy density, cycle life, and efficiency can be obtained. More specifically, the average particle diameter of the silicon oxide (c) is preferably 1/2 or less of the average particle diameter of the graphite (a), and the average particle diameter of the silicon (b) is preferably 1/2 or less of the average particle diameter of the silicon oxide (c). More specifically, the average particle size of the silicon (b) may be, for example, 20 μm or less, and preferably 15 μm or less.
As the negative electrode active material, graphite whose surface is covered with a low crystalline carbon material can be used. By covering the surface of the graphite with the low crystalline carbon material, even when graphite having a high energy density and high conductivity is used as the negative electrode active material, the reaction between the negative electrode active material and the electrolyte can be suppressed. Therefore, by using graphite coated with a low crystalline carbon material as the negative electrode active material, the capacity retention rate of the battery can be improved, and the battery capacity can be improved.
1300cm in Raman spectrum of low crystalline carbon material covering graphite surface based on laser Raman analysis -1 ～1400cm -1 D peak intensity I of the D peak D Relative to the length at 1550cm -1 ～1650cm -1 Range (d) produces an intensity of the G peak I G Ratio of (A to (B)) D /I G Preferably 0.08 or more and 0.5 or less.
Generally, carbon materials with high crystallinity exhibit low I D /I G Carbon having a low value and crystallinity shows a high I D /I G The value is obtained. If I D /I G When the amount is 0.08 or more, the reaction between graphite and the electrolyte can be suppressed even when the battery is operated at a high voltage, and the capacity retention rate of the battery can be improved. If I D /I G When the amount is 0.5 or less, the battery capacity can be improved. In addition, I D /I G More preferably 0.1 or more and 0.4 or less.
For the laser raman analysis of the low crystalline carbon material, for example, an argon ion laser raman analyzer may be used. In the case of a material having a large laser absorption such as a carbon material, the laser is absorbed from the surface up to several 10 nm. Therefore, by laser raman analysis of graphite whose surface is covered with the low-crystalline carbon material, information of the low-crystalline carbon material disposed on the surface can be substantially obtained.
I D Value or I G The value can be obtained from, for example, a laser raman spectrum measured under the following conditions.
Laser Raman spectrometer: ramanor T-64000(Jobin Yvon/Aidang products Co., Ltd.)
Measurement mode: macroscopic Raman
Measurement configuration: 60 DEG C
Beam diameter: 100 μm
Light source: ar + laser/514.5 nm
Laser power: 10mW
Diffraction grating: single 600gr/mm
Dispersing: single 21A/mm
Slit: 100 μm
A detector: CCD/Jobin Yvon1024256
The graphite coated with the low crystalline carbon material can be obtained by, for example, coating particulate graphite with the low crystalline carbon material. Average particle diameter (volume average: D) of graphite particles 50 ) Preferably 5 μm or more and 30 μm or less. The graphite preferably has crystallinity, I of graphite D /I G The value is more preferably 0.01 or more and 0.08 or less.
The thickness of the low crystalline carbon material is preferably 0.01 μm or more and 5 μm or less, and more preferably 0.02 μm or more and 1 μm or less.
Average particle diameter (D) 50 ) Can be used forThe particle size is measured using, for example, a microtrac mt3300EX (manufactured by japan ltd.) which is a laser diffraction/scattering particle size distribution measuring apparatus.
The low crystalline carbon material can be formed on the surface of graphite by, for example, a vapor phase method in which a hydrocarbon such as propane or acetylene is thermally decomposed to deposit carbon. The low-crystalline carbon material can be formed by, for example, a method in which pitch, tar, or the like is attached to the surface of graphite and the graphite is fired at 800 to 1500 ℃.
In the crystal structure of graphite, the layer spacing d of 002 planes 002 Preferably 0.33nm or more and 0.34nm or less, more preferably 0.333nm or more and 0.337nm or less, and further preferably 0.336nm or less. The highly crystalline graphite has a high lithium storage capacity and can improve the charge-discharge efficiency.
The layer spacing of graphite can be measured by, for example, X-ray diffraction.
The specific surface area of the graphite coated with the low-crystalline carbon material is, for example, 0.01 to 20m 2 A preferred range is 0.05 to 10 m/g 2 A concentration of 0.1 to 5m 2 A specific ratio of 0.2 to 3 m/g 2 (ii) in terms of/g. The specific surface area of the graphite coated with the low-crystalline carbon was set to 0.01m 2 More than g, since the lithium ions are easily inserted and released smoothly, the resistance can be further reduced. The specific surface area of the graphite coated with the low-crystalline carbon was set to 20m 2 The concentration of the active material is not more than g, and decomposition of the electrolytic solution can be further suppressed, and elution of the constituent elements of the active material into the electrolytic solution can be further suppressed.
The graphite forming the substrate is preferably highly crystalline graphite, and examples thereof include, but are not particularly limited to, artificial graphite and natural graphite. For example, coal tar, pitch coke, or a phenol resin can be used as the material of the low crystalline carbon and mixed with the high crystalline carbon. A material containing low-crystalline carbon is mixed in an amount of 5 to 50 mass% relative to the high-crystalline carbon to prepare a mixture. The mixture is heated to 150 to 300 ℃ and then further heat-treated in the range of 600 to 1500 ℃. Thus, surface-treated graphite having a surface coated with low-crystalline carbon can be obtained. The heat treatment is preferably performed in an inert gas atmosphere such as argon, helium, or nitrogen.
The negative electrode active material may contain other active materials in addition to graphite coated with the low crystalline carbon material.
The binder for the negative electrode is not particularly limited, and examples thereof include polyvinylidene fluoride (PVdF), a vinylidene fluoride-hexafluoropropylene copolymer, a vinylidene fluoride-tetrafluoroethylene copolymer, a styrene-butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, and polyamide imide.
The content of the binder for a negative electrode is preferably in the range of 1 to 30% by mass, more preferably 2 to 25% by mass, based on the total amount of the negative electrode active material and the binder for a negative electrode. When the amount is not less than the lower limit, the adhesion between the negative electrode active materials or between the negative electrode active material and the current collector is improved, and the cycle characteristics are improved. When the amount is equal to or less than the upper limit, the ratio of the negative electrode active material is increased, and the negative electrode capacity can be increased.
The negative electrode current collector 4 is not particularly limited, and is preferably composed of one or two or more selected from aluminum, nickel, copper, silver, and alloys thereof, and stainless steel, from the viewpoint of electrochemical stability. Examples of the shape of the negative electrode current collector 4 include foil, flat plate, and mesh.
The negative electrode 12 can be produced by, for example, forming a negative electrode active material layer 2 containing a negative electrode active material and a binder for a negative electrode on a negative electrode current collector 4. Examples of the method for forming the negative electrode active material layer 2 include a doctor blade method, a die coating method, a CVD method, a sputtering method, and the like. The negative electrode current collector 4 may be prepared by forming the negative electrode active material layer 2 in advance, and then forming a thin film of aluminum, nickel, or an alloy thereof by a method such as vapor deposition or sputtering.
(separation layer)
The separation layer 5 constituting the secondary battery 10 according to the present embodiment may use, for example, a spacer. Examples of the spacer include woven fabrics; non-woven fabrics; polyolefin films such as polyethylene and polypropylene, and porous polymer films such as polyimide films and porous polyvinylidene fluoride films; ion conductive polymer electrolyte membranes, and the like. They may be used alone or in combination.
When a solid electrolyte is used as the ion conductor, the solid electrolyte may also serve as the separation layer 5.
(ion conductor)
Examples of the ionic conductor constituting the secondary battery 10 according to the present embodiment include an electrolytic solution containing a supporting salt and a nonaqueous electrolytic solvent, and a solid electrolyte.
The nonaqueous electrolytic solvent preferably contains a cyclic carbonate and/or a chain carbonate.
Since cyclic carbonates or chain carbonates have a large relative dielectric constant, addition of these carbonates improves the dissociation of the supporting salt, and thus sufficient conductivity is easily imparted. In addition, cyclic carbonates and chain carbonates have high voltage resistance and conductivity, and therefore are suitable for mixing with fluorine-containing phosphate esters. Further, by selecting a material having an effect of reducing the viscosity of the electrolyte, the ion mobility in the electrolyte can be increased.
The cyclic carbonate is not particularly limited, and examples thereof include Ethylene Carbonate (EC), Propylene Carbonate (PC), Butylene Carbonate (BC), Vinylene Carbonate (VC), and the like.
Further, the cyclic carbonate includes a fluorinated cyclic carbonate. Examples of the fluorinated cyclic carbonate include compounds obtained by substituting a part or all of hydrogen atoms of Ethylene Carbonate (EC), Propylene Carbonate (PC), Butylene Carbonate (BC), Vinylene Carbonate (VC), or the like with fluorine atoms.
More specifically, as the fluorinated cyclic carbonate, 4-fluoro-1, 3-dioxolan-2-one, (cis or trans) 4, 5-difluoro-1, 3-dioxolan-2-one, 4-difluoro-1, 3-dioxolan-2-one, 4-fluoro-5-methyl-1, 3-dioxolan-2-one, and the like can be used.
Among the above, the cyclic carbonate is preferably ethylene carbonate, propylene carbonate, or a compound obtained by partially fluorinating these compounds, and more preferably ethylene carbonate, from the viewpoint of voltage resistance and conductivity. The cyclic carbonate may be used alone or in combination of two or more.
From the viewpoint of the effect of increasing the dissociation degree of the supporting salt and the effect of improving the conductivity of the electrolytic solution, the content of the cyclic carbonate in the nonaqueous electrolytic solvent is preferably 0.1% by volume or more, more preferably 5% by volume or more, further preferably 10% by volume or more, and particularly preferably 15% by volume or more. From the viewpoint of the effect of increasing the dissociation degree of the supporting salt and the effect of improving the conductivity of the electrolytic solution, the content of the cyclic carbonate in the nonaqueous electrolytic solvent is preferably 70% by volume or less, more preferably 50% by volume or less, and still more preferably 40% by volume or less.
The chain carbonate is not particularly limited, and examples thereof include dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), and the like.
The chain carbonate includes a fluorinated chain carbonate. Examples of the fluorinated chain carbonate include compounds having a structure in which some or all of hydrogen atoms are replaced by fluorine atoms, such as Ethyl Methyl Carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC).
The fluorinated chain carbonate is, more specifically, examples thereof include bis (fluoroethyl) carbonate, 3-fluoropropylmethyl carbonate, 3, 3, 3-trifluoropropylmethyl carbonate, 2, 2, 2-trifluoroethyl methyl carbonate, 2, 2, 2-trifluoroethyl ethyl carbonate, monofluoromethyl methyl carbonate, methyl 2, 2, 3, 3-tetrafluoropropyl carbonate, ethyl 2, 2, 3, 3-tetrafluoropropyl carbonate, bis (2, 2, 3, 3-tetrafluoropropyl) carbonate, bis (2, 2, 2-trifluoroethyl) carbonate, 1-monofluoroethyl ethyl carbonate, 1-monofluoroethyl methyl carbonate, 2-monofluoroethyl methyl carbonate, bis (1-monofluoroethyl) carbonate, bis (2-monofluoroethyl) carbonate, and bis (monofluoromethyl) carbonate.
Among these, dimethyl carbonate, 2, 2, 2-trifluoroethyl methyl carbonate, monofluoromethyl methyl carbonate, methyl 2, 2, 3, 3-tetrafluoropropyl carbonate, and the like are preferable from the viewpoint of voltage resistance and conductivity. One kind of chain carbonate may be used alone, or two or more kinds may be used in combination.
When the number of carbon atoms of the substituent added to the "-OCOO-" structure is small, the chain carbonate has an advantage of low viscosity. On the other hand, if the carbon number is too large, the viscosity of the electrolyte solution may be high, and the conductivity of Li ions may be reduced. For this reason, the total number of carbons of 2 substituents added to the "-OCOO-" structure of the chain carbonate is preferably 2 or more and 6 or less. Further, when the substituent added to the "— OCOO-" structure contains a fluorine atom, the oxidation resistance of the electrolyte solution is improved. For this reason, the chain carbonate is preferably a fluorinated chain carbonate represented by the following formula (2).
C n H 2n+1-1 F 1 -OCOO-C m H 2m+1-k F k (2)
(in the formula (2), n is 1, 2 or 3, m is 1, 2 or 3, 1 is an arbitrary integer of 0 to 2n +1, k is an arbitrary integer of 0 to 2m +1, and at least one of l and k is an integer of 1 or more.)
In the fluorinated chain carbonate represented by the above formula (2), when the amount of fluorine substitution is small, the fluorinated chain carbonate reacts with a high-potential positive electrode, and thus the capacity retention rate of the battery may decrease or gas may be generated. On the other hand, when the fluorine substitution amount is too large, the compatibility of the chain carbonate with other solvents may be lowered or the boiling point of the chain carbonate may be lowered. For this reason, the fluorine substitution amount is preferably 1% or more and 90% or less, more preferably 5% or more and 85% or less, and further preferably 10% or more and 80% or less. In other words, l, m, and n in the above formula (2) preferably satisfy the following relational expression.
0.01≤(1+k)/(2n+2m+2)≤0.9
The chain carbonate has an effect of reducing the viscosity of the electrolyte solution, and can increase the conductivity of the electrolyte solution. From these viewpoints, the content of the chain carbonate in the nonaqueous electrolytic solvent is preferably 5% by volume or more, more preferably 10% by volume or more, and further preferably 15% by volume or more. The content of the chain carbonate in the nonaqueous electrolytic solvent is preferably 90% by volume or less, more preferably 80% by volume or less, and still more preferably 70% by volume or less.
The content of the fluorinated chain carbonate is not particularly limited, and is preferably 0.1 vol% or more and 70 vol% or less in the nonaqueous electrolytic solvent. When the content of the fluorinated chain carbonate in the nonaqueous electrolytic solvent is not less than the lower limit, the viscosity of the electrolytic solution can be reduced and the conductivity can be improved. Further, an effect of improving oxidation resistance can be obtained. Further, when the content of the fluorinated chain carbonate in the nonaqueous electrolytic solvent is not more than the upper limit value, the conductivity of the electrolytic solution can be kept high. The content of the fluorinated chain carbonate in the nonaqueous electrolytic solvent is more preferably 1% by volume or more, still more preferably 5% by volume or more, and particularly preferably 10% by volume or more. The content of the fluorinated chain carbonate in the nonaqueous electrolytic solvent is more preferably 65% by volume or less, still more preferably 60% by volume or less, and particularly preferably 55% by volume or less.
The nonaqueous electrolytic solvent may contain a fluorine-containing phosphate represented by the following formula (3).
[ solution 1]
Figure BDA0002163473440000211
(in the above formula (3), R 1 、R 2 And R 3 Each independently is substituted or unsubstituted alkyl, R 1 、R 2 And R 3 At least 1 of which is a fluoroalkyl group. )
The nonaqueous electrolytic solvent may contain a fluorine-containing chain ether represented by the following formula (4).
A-O-B (4)
(in the above formula (4), A and B are each independently a substituted or unsubstituted alkyl group, and at least 1 of A and B is a fluoroalkyl group.)
By using the nonaqueous electrolytic solvent, the volume expansion of the secondary battery 10 can be suppressed, and the capacity retention rate can be improved. The reason is not clear, but it is presumed that: in the electrolytic solution containing these, the fluorine-containing phosphate ester and the fluorine-containing ether function as an oxidation-resistant solvent, and the acid anhydride forms a reaction product on the electrode, whereby the reaction of the electrolytic solution can be suppressed, and the volume expansion can be suppressed. Further, it is considered that: by causing these to act synergistically, the cycle characteristics can be made to be more favorable characteristics. This is a characteristic that the effect is more remarkably exhibited in a long-term charge-discharge cycle in which decomposition of an electrolytic solution becomes a serious problem, in use or after storage of a secondary battery under high-temperature conditions, and in the case of using a high-potential positive electrode active material.
The content of the fluorine-containing phosphate represented by the formula (3) in the nonaqueous electrolytic solvent is not particularly limited, and is preferably 5% by volume or more and 95% by volume or less in the nonaqueous electrolytic solvent. When the content of the fluorine-containing phosphate ester in the nonaqueous electrolytic solvent is not less than the lower limit, the effect of improving the withstand voltage is further improved. When the content of the fluorine-containing phosphate ester in the nonaqueous electrolytic solvent is not more than the upper limit, the ion conductivity of the electrolytic solution is improved, and the charge/discharge rate of the battery is further improved. The content of the fluorinated phosphate in the nonaqueous electrolytic solvent is more preferably 10 vol% or more. The content of the fluorinated phosphate in the nonaqueous electrolytic solvent is more preferably 70% by volume or less, still more preferably 60% by volume or less, particularly preferably 59% by volume or less, and still more particularly preferably 55% by volume or less.
In the fluorine-containing phosphoric ester represented by the above formula (3), R 1 、R 2 And R 3 Each independently is substituted or unsubstituted alkyl, R 1 、R 2 And R 3 At least 1 of which is a fluoroalkyl group. Fluoroalkyl refers to an alkyl group having at least 1 fluorine atom. Alkyl radical R 1 、R 2 And R 3 Each independently of the other, is preferably 1 to 4 carbon atoms, more preferably 1 to 3 carbon atoms. This is because: when the number of carbon atoms of the alkyl group is not more than the upper limit, the viscosity of the electrolyte solution can be suppressed from increasing, the electrolyte solution can easily infiltrate into pores in the electrode or the separator, the ion conductivity is improved, and the current value is improved in terms of the charge and discharge characteristics of the battery.
In the above formula (3), R is preferably R 1 、R 2 And R 3 Are all fluorine-containing alkyl groups.
Furthermore, R is preferred 1 、R 2 And R 3 At least 1 of the (a) fluorine-containing alkyl groups is a fluorine-containing alkyl group in which at least 50% of hydrogen atoms of the corresponding unsubstituted alkyl group are substituted with fluorine atoms.
Furthermore, R is more preferable 1 、R 2 And R 3 Are all fluorine-containing alkyl, and the R 1 、R 2 And R 3 Is a fluoroalkyl group in which 50% or more of the hydrogen atoms of the corresponding unsubstituted alkyl group are substituted with fluorine atoms.
This is because: when the content of fluorine atoms is high, the voltage resistance is further improved, and even when a positive electrode active material that operates at a potential of 4.5V or more with respect to lithium metal is used, the deterioration of the battery capacity after the cycle can be further reduced.
Further, the ratio of the fluorine atom in the substituent group containing a hydrogen atom in the fluoroalkyl group is more preferably 55% or more.
Furthermore, R 1 ～R 3 The compound may have a substituent other than the fluorine atom, and examples of the substituent include at least 1 selected from the group consisting of an amino group, a carboxyl group, a hydroxyl group, a cyano group, and a halogen atom (e.g., a chlorine atom and a bromine atom). The carbon number is a concept including a substituent.
Examples of the fluorine-containing phosphate ester include tris (trifluoromethyl) phosphate, tris (trifluoroethyl) phosphate, tris (tetrafluoropropyl) phosphate, tris (pentafluoropropyl) phosphate, tris (heptafluorobutyl) phosphate, and tris (octafluoropentyl) phosphate.
Examples of the fluorine-containing phosphate ester include trifluoroethyl dimethyl phosphate, bis (trifluoroethyl) methyl phosphate, bis trifluoroethyl ethyl phosphate, pentafluoropropyl dimethyl phosphate, heptafluorobutyl dimethyl phosphate, trifluoroethyl methyl ethyl phosphate, pentafluoropropyl methyl ethyl phosphate, heptafluorobutyl methyl ethyl phosphate, trifluoroethyl methyl propyl phosphate, pentafluoropropyl methyl propyl phosphate, heptafluorobutyl methyl propyl phosphate, trifluoroethyl methyl butyl phosphate, pentafluoropropyl methyl butyl phosphate, heptafluorobutyl methyl butyl phosphate, trifluoroethyl diethyl phosphate, pentafluoropropyl diethyl phosphate, heptafluorobutyl diethyl phosphate, trifluoroethyl ethyl propyl phosphate, pentafluoropropyl ethyl propyl phosphate, heptafluorobutyl ethyl propyl phosphate, trifluoroethyl butyl phosphate, pentafluoropropyl ethyl butyl phosphate, heptafluorobutyl ethyl butyl phosphate, heptafluorobutyl ethyl butyl phosphate, and the like, Trifluoroethyl dipropyl phosphate, pentafluoropropyl dipropyl phosphate, heptafluorobutyl dipropyl phosphate, trifluoroethyl propyl butyl phosphate, pentafluoropropyl propyl butyl phosphate, heptafluorobutyl propyl butyl phosphate, trifluoroethyl dibutyl phosphate, pentafluoropropyl dibutyl phosphate, heptafluorobutyl dibutyl phosphate, and the like.
Examples of tris (tetrafluoropropyl) phosphate include tris (2, 2, 3, 3-tetrafluoropropyl) phosphate.
Examples of tris (pentafluoropropyl) phosphate include tris (2, 2, 3, 3, 3-pentafluoropropyl) phosphate.
Examples of tris (trifluoroethyl) phosphate include tris (2, 2, 2-trifluoroethyl) phosphate (hereinafter, also abbreviated as PTTFE).
Examples of the tris (heptafluorobutyl) phosphate include tris (1H, 1H-heptafluorobutyl) phosphate and the like.
Examples of tris (octafluoropentyl) phosphate include tris (1H, 1H, 5H-octafluoropentyl) phosphate.
Among these, tris (2, 2, 2-trifluoroethyl) phosphate represented by the following formula (3-1) is preferable in that the effect of suppressing decomposition of the electrolyte solution at a high potential is high.
The fluorine-containing phosphoric acid ester may be used singly or in combination of two or more.
[ solution 2]
Figure BDA0002163473440000241
The nonaqueous electrolytic solvent may contain a carboxylic acid ester.
The carboxylic acid ester is not particularly limited, and examples thereof include ethyl acetate, methyl propionate, ethyl formate, ethyl propionate, methyl butyrate, ethyl butyrate, methyl acetate, and methyl formate.
The carboxylic acid ester also includes fluorinated carboxylic acid esters, and examples of the fluorinated carboxylic acid ester include compounds having a structure in which a part or all of hydrogen atoms of ethyl acetate, methyl propionate, ethyl formate, ethyl propionate, methyl butyrate, ethyl butyrate, methyl acetate, or methyl formate is substituted with fluorine atoms.
Specific examples of the fluorinated carboxylic acid ester include ethyl pentafluoropropionate, ethyl 3, 3, 3-trifluoropropionate, methyl 2, 2, 3, 3-tetrafluoropropionate, 2, 2-difluoroethyl acetate, methyl heptafluoroisobutyrate, methyl 2, 3, 3, 3-tetrafluoropropionate, methyl pentafluoropropionate, methyl 2- (trifluoromethyl) -3, 3, 3-trifluoropropionate, ethyl heptafluorobutyrate, methyl 3, 3, 3-trifluoropropionate, 2, 2, 2-trifluoroethyl acetate, isopropyl trifluoroacetate, t-butyl trifluoroacetate, ethyl 4, 4, 4-trifluorobutyrate, methyl 4, 4-trifluorobutyrate, 2, 2-difluoroacetate, ethyl difluoroacetate, n-butyl trifluoroacetate, 2, 2, 3, 3-tetrafluoropropyl trifluoroacetate, Ethyl 3- (trifluoromethyl) butyrate, methyl tetrafluoro-2- (methoxy) propionate, 3, 3, 3-trifluoropropionate, 3, 3-trifluoropropyl 3, 3, 3-difluoroacetate, 2, 3, 3-tetrafluoropropyl trifluoroacetate, 1H-heptafluorobutyl acetate, methyl heptafluorobutyrate, ethyl trifluoroacetate, and the like.
Among these, ethyl propionate, methyl acetate, methyl 2, 2, 3, 3-tetrafluoropropionate, and 2, 2, 3, 3-tetrafluoropropyl trifluoroacetate are preferable as the carboxylate from the viewpoint of withstand voltage, boiling point, and the like. The carboxylic acid ester has an effect of reducing the viscosity of the electrolyte solution, similarly to the chain carbonate. Therefore, for example, a carboxylic acid ester may be used instead of the chain carbonate, or may be used in combination with the chain carbonate.
When the number of carbon atoms of the substituent group added to the "-COO-" structure is small, the linear carboxylic acid ester has an advantage of low viscosity, but tends to have a low boiling point. The chain carboxylic acid ester having a low boiling point is sometimes gasified at the time of high-temperature operation of the battery. On the other hand, if the carbon number is too large, the viscosity of the electrolyte solution may be increased, and the conductivity may be decreased. For this reason, the total number of carbons of the 2 substituents added to the "-COO-" structure of the chain carboxylic ester is preferably 3 or more and 8 or less. Further, when the substituent added to the "-COO-" structure contains a fluorine atom, the oxidation resistance of the electrolytic solution can be improved. For this reason, the chain carboxylic acid ester is preferably a fluorinated chain carboxylic acid ester represented by the following formula (5).
C n H 2n+1-1 F 1 -COO-C m H 2m+1-k F k (5)
(in the above formula (5), n is 1, 2, 3 or 4, m is 1, 2, 3 or 4, 1 is an arbitrary integer of 0 to 2n +1, k is an arbitrary integer of 0 to 2m +1, and at least one of l and k is an integer of 1 or more.)
In the fluorinated chain carboxylic acid ester represented by the above formula (5), when the amount of fluorine substitution is small, the fluorinated chain carboxylic acid ester reacts with the high-potential positive electrode, and thus the capacity retention rate of the battery may decrease or gas may be generated. On the other hand, when the fluorine substitution amount is too large, the compatibility of the chain carboxylic ester with other solvents may be lowered or the boiling point of the fluorinated chain carboxylic ester may be lowered. For this reason, the fluorine substitution amount is preferably 1% or more and 90% or less, more preferably 10% or more and 85% or less, and further preferably 20% or more and 80% or less. In other words, l, m, and n in the above formula (5) preferably satisfy the following relational expression.
0.01≤(1+k)/(2n+2m+2)≤0.9
The content of the carboxylic acid ester in the nonaqueous electrolytic solvent is preferably 0.1% by volume or more, more preferably 0.2% by volume or more, further preferably 0.5% by volume or more, and particularly preferably 1% by volume or more. The content of the carboxylic acid ester in the nonaqueous electrolytic solvent is preferably 50% by volume or less, more preferably 20% by volume or less, still more preferably 15% by volume or less, and particularly preferably 10% by volume or less. When the content of the carboxylic acid ester is not less than the lower limit, the low-temperature characteristics can be further improved, and the conductivity can be further improved. Further, by setting the content of the carboxylic acid ester to the upper limit or less, it is possible to reduce the phenomenon that the vapor pressure becomes excessively high when the battery is left at a high temperature.
The content of the fluorinated chain carboxylic acid ester is not particularly limited, and is preferably 0.1 vol% or more and 50 vol% or less in the nonaqueous electrolytic solvent. When the content of the fluorinated chain carboxylic acid ester in the nonaqueous electrolytic solvent is not less than the lower limit, the viscosity of the electrolytic solution can be reduced and the conductivity can be improved. Further, an effect of improving oxidation resistance can be obtained. When the content of the fluorinated chain carboxylic acid ester in the nonaqueous electrolytic solvent is not more than the upper limit, the conductivity of the electrolytic solution can be kept high, and the compatibility of the electrolytic solution can be ensured. The content of the fluorinated chain carboxylic acid ester in the nonaqueous electrolytic solvent is more preferably 1% by volume or more, still more preferably 5% by volume or more, and particularly preferably 10% by volume or more. The content of the fluorinated chain carboxylic acid ester in the nonaqueous electrolytic solvent is more preferably 45 vol% or less, still more preferably 40 vol% or less, and particularly preferably 35 vol% or less.
The nonaqueous electrolytic solvent may contain a dialkylene carbonate represented by the following formula (6) in addition to the fluorine-containing phosphate. The oxidation resistance of the dialkylene carbonate is equal to or slightly higher than that of the chain carbonate, and therefore, the voltage resistance of the electrolytic solution can be improved.
[ solution 3]
Figure BDA0002163473440000261
(R 4 And R 6 Each independently represents a substituted or unsubstituted alkyl group. R 5 Represents a substituted or unsubstituted alkylene group. ).
In the formula (6), the alkyl group includes a straight-chain or branched-chain alkyl group, and the number of carbon atoms is preferably 1 to 6, more preferably 1 to 4. The alkylene group is a divalent saturated hydrocarbon group, and includes a straight-chain or branched alkylene group, and the number of carbon atoms is preferably 1 to 4, and more preferably 1 to 3.
Examples of the dialkylene carbonate represented by the above formula (6) include 1, 2-bis (methoxycarbonyloxy) ethane, 1, 2-bis (ethoxycarbonyloxy) ethane, 1, 2-bis (methoxycarbonyloxy) propane and 1-ethoxycarbonyloxy-2-methoxycarbonyloxyethane. Among these, 1, 2-bis (methoxycarbonyloxy) ethane is preferable.
The content of the dialkylene carbonate in the nonaqueous electrolytic solvent is preferably 0.1% by volume or more, more preferably 0.5% by volume or more, further preferably 1% by volume or more, and particularly preferably 1.5% by volume or more. The content ratio of the dialkylene carbonate in the nonaqueous electrolytic solvent is preferably 70% by volume or less, more preferably 60% by volume or less, further preferably 50% by volume or less, and particularly preferably 40% by volume or less.
Dialkylene carbonates are materials with low dielectric constants. Therefore, for example, it can be used instead of or in combination with chain carbonates.
The nonaqueous electrolytic solvent may contain a chain ether.
The chain ether is not particularly limited, and examples thereof include 1, 2-ethoxyethane (DEE) and ethoxymethoxyethane (EME).
The chain ether may include a halogenated chain ether such as a fluorinated ether. The halogenated chain ether has high oxidation resistance and is preferably used for a positive electrode operating at a high potential.
The chain ether has an effect of reducing the viscosity of the electrolyte solution, similarly to the chain carbonate. Therefore, for example, a chain ether may be used instead of or in combination with a chain carbonate or a carboxylic acid ester.
The content of the chain ether is not particularly limited, and is preferably 0.1 vol% or more and 70 vol% or less in the nonaqueous electrolytic solvent. If the content of the chain ether in the nonaqueous electrolytic solvent is 0.1 vol% or more, the viscosity of the electrolytic solution can be reduced, and the conductivity can be improved. Further, an effect of improving oxidation resistance can be obtained. Further, when the content of the chain ether in the nonaqueous electrolytic solvent is 70 vol% or less, the conductivity of the electrolytic solution can be kept high, and the compatibility of the electrolytic solution can be ensured. The content of the chain ether in the nonaqueous electrolytic solvent is more preferably 1% by volume or more, still more preferably 5% by volume or more, and particularly preferably 10% by volume or more. The content of the chain ether in the nonaqueous electrolytic solvent is more preferably 65% by volume or less, still more preferably 60% by volume or less, and particularly preferably 55% by volume or less.
The solvent for nonaqueous electrolysis may contain a sulfone compound represented by the following formula (7).
[ solution 4]
Figure BDA0002163473440000271
(in the formula, R 1 And R 2 Each independently represents a substituted or unsubstituted alkyl group. R 1 Optionally bonded to the carbon atom of R2 by a single or double bond to form a cyclic structure. )
In the sulfone compound represented by the above formula (7), R 1 N is a carbon number of 1 、R 2 N is a carbon number of 2 Are each preferably 1. ltoreq. n 1 ≤12、1≤n 2 12 or less, more preferably 1 or less n 1 ≤6、1≤n 2 6 or less, and more preferably 1 or less n 1 ≤3、1≤n 2 Less than or equal to 3. The alkyl group includes linear, branched or cyclic alkyl groups.
At R 1 And R 2 Examples of the substituent include an alkyl group having 1 to 6 carbon atoms (e.g., methyl, ethyl, propyl, isopropyl, butyl, isobutyl), and an aryl group having 6 to 10 carbon atoms (e.g., phenyl, naphthyl).
In one embodiment, the sulfone compound is more preferably a cyclic sulfone compound represented by the following formula (7-1).
[ solution 5]
Figure BDA0002163473440000281
(wherein R is 3 Represents a substituted or unsubstituted alkylene group. )
At R 3 In the above step, the number of carbon atoms in the alkylene group is preferably 4 to 9, more preferably 4 to 6.
At R 3 In the above formula, examples of the substituent include an alkyl group having 1 to 6 carbon atoms (e.g., methyl group, ethyl group, etc.),Ethyl group, propyl group, isopropyl group, butyl group), a halogen atom (e.g., chlorine atom, bromine atom, fluorine atom), and the like.
The cyclic sulfone compound is more preferably a compound represented by the following formula (7-2).
[ solution 6]
Figure BDA0002163473440000282
(wherein m is an integer of 1 to 6.)
In the formula (7-2), m is an integer of 1 to 6, preferably an integer of 1 to 3.
Examples of the cyclic sulfone compound represented by the above formula (7-1) include tetramethylene sulfone (sulfolane), pentamethylene sulfone, and hexamethylene sulfone. Examples of the cyclic sulfone compound having a substituent include 3-methylsulfolane and 2, 4-dimethylsulfolane.
Further, the sulfone compound may be a chain sulfone compound. Examples of the chain sulfone compound include ethyl methyl sulfone, ethyl isopropyl sulfone, ethyl isobutyl sulfone, dimethyl sulfone, and diethyl sulfone. Among these, ethyl methyl sulfone, ethyl isopropyl sulfone, and ethyl isobutyl sulfone are preferable.
The sulfone compound has compatibility with other solvents such as a fluorinated ether compound and the like, and has a high dielectric constant, and therefore, the dissolution/dissociation of the lithium salt is excellent. The sulfone compound may be used alone in 1 kind, or in a mixture of 2 or more kinds.
In the case where the sulfone compound is contained, it is preferably 1% by volume or more and 75% by volume or less, more preferably 5% by volume or more and 50% by volume or less in the nonaqueous electrolytic solvent. When the sulfone compound is not less than the lower limit, the compatibility of the electrolyte solution is improved. If the content of the sulfone compound is too large, the viscosity of the electrolyte solution becomes high, and in particular, the capacity of the charge-discharge cycle characteristics at room temperature may be reduced.
The nonaqueous electrolyte solvent may contain an acid anhydride. The content of the acid anhydride contained in the nonaqueous electrolytic solvent is not particularly limited, and is preferably 0.01 mass% or more and less than 10 mass%, and more preferably 0.1 mass% or more and 5 mass% or less in the nonaqueous electrolytic solvent. When the content of the acid anhydride in the nonaqueous electrolytic solvent is 0.01% by mass or more, the effect of improving the capacity retention rate and the effect of suppressing the generation of gas due to decomposition of the electrolytic solution can be obtained. The content of the acid anhydride in the nonaqueous electrolytic solvent is more preferably 0.1% by mass or more. Further, if the content of the acid anhydride in the nonaqueous electrolytic solvent is less than 10% by mass, a good capacity retention rate can be maintained, and the amount of gas generated by decomposition of the acid anhydride can be suppressed. The content of the acid anhydride in the nonaqueous electrolytic solvent is more preferably 5% by mass or less. The content of the acid anhydride in the nonaqueous electrolytic solvent is more preferably 0.5% by mass or more, and particularly preferably 0.8% by mass or more. The content of the acid anhydride in the nonaqueous electrolytic solvent is more preferably 3% by mass or less, and particularly preferably 2% by mass or less.
Examples of the acid anhydride include carboxylic acid anhydrides, sulfonic acid anhydrides, and anhydrides of carboxylic acids and sulfonic acids.
It can be considered that: the acid anhydride in the electrolyte solution has the effect of forming a reaction product on the electrode, suppressing the volume expansion of the battery associated with charge and discharge, and improving the cycle characteristics. It is presumed that the acid anhydride as described above is bonded to moisture in the electrolyte solution, and therefore, the generation of gas due to moisture is also suppressed.
Examples of the acid anhydride include a chain acid anhydride represented by the following formula (8) and a cyclic acid anhydride represented by the following formula (9).
[ solution 7]
Figure BDA0002163473440000301
(in the above formula (8), 2X 1 Each independently is a carbonyl (-C (═ O) -) or sulfonyl (-S (═ O) 2 -)，R 1 And R 2 Each independently an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, an aryl group having 6 to 18 carbon atoms or an arylalkyl group having 7 to 20 carbon atoms,R 1 and R 2 At least 1 hydrogen atom in (a) is optionally substituted with a halogen atom. )
[ solution 8]
Figure BDA0002163473440000302
(in the above formula (9), 2X 2 Each independently is a carbonyl (-C (═ O) -) or sulfonyl (-S (═ O) 2 -)，R 3 Is alkylene group having 1 to 10 carbon atoms, alkenylene group having 2 to 10 carbon atoms, arylene group having 6 to 12 carbon atoms, cycloalkylene group having 3 to 12 carbon atoms, cycloalkenylene group having 3 to 12 carbon atoms or heterocycloalkylene group having 3 to 10 carbon atoms, R 3 At least 1 hydrogen atom in (a) is optionally substituted with a halogen atom. )
In the above formulae (8) and (9), R is represented by 1 、R 2 Or R 3 The groups shown are described below.
In the formula (8), the alkyl group and the alkenyl group may be each a straight chain or a branched chain, and the number of carbon atoms is usually 1 to 10, preferably 1 to 8, and more preferably 1 to 5.
In the formula (8), the carbon number of the cycloalkyl group is preferably 3 to 10, more preferably 3 to 6.
In the formula (8), the carbon number of the aryl group is preferably 6 to 18, more preferably 6 to 12. Examples of the aryl group include a phenyl group and a naphthyl group.
In the formula (8), the number of carbon atoms in the arylalkyl group is preferably 7 to 20, more preferably 7 to 14. Examples of the arylalkyl group include a benzyl group, a phenylethyl group, a naphthylmethyl group and the like.
In the above formula (8), R 1 And R 2 Each independently is more preferably an alkyl group having 1 to 3 carbon atoms or a phenyl group.
In the formula (9), the alkylene group and the alkenylene group may be each a straight chain or a branched chain, and the number of carbon atoms is usually 1 to 10, preferably 1 to 8, and more preferably 1 to 5.
In the formula (9), the carbon number of the arylene group is preferably 6 to 20, more preferably 6 to 12. Examples of the arylene group include a phenylene group, a naphthylene group, and a biphenylene group.
In the formula (9), the carbon number of the cycloalkylene group is usually 3 to 12, preferably 3 to 10, and more preferably 3 to 8. The cycloalkylene group may be a monocyclic group, or may have a plurality of ring structures like a bicycloalkylene group.
In the formula (9), the number of carbon atoms of the cycloalkenylene group is usually 3 to 12, preferably 3 to 10, and more preferably 3 to 8. The cycloalkenylene group may be a monocyclic group, or may have a plurality of ring structures in which at least 1 ring has an unsaturated bond, like a bicycloalkenyl group. Examples of the cycloalkenylene group include a 2-valent group formed from cyclohexene, bicyclo [2.2.1] heptene, bicyclo [2.2.2] octene and the like.
In the formula (9), the heterocycloalkylene group represents a 2-valent group in which at least one carbon atom in the ring of the cycloalkylene group is substituted with 1 or 2 or more kinds of hetero atoms selected from sulfur, oxygen, nitrogen and the like. The heterocycloalkylene group is preferably a 3-to 10-membered ring, more preferably a 4-to 8-membered ring, and further preferably a 5-or 6-membered ring.
In the above formula (9), R 3 More preferably an alkylene group having 1 to 3 carbon atoms, an alkenylene group having 2 or 3 carbon atoms, a cyclohexylene group, a cyclohexenylene group or a phenylene group.
The anhydride may be partially halogenated. Examples of the halogen atom include chlorine, iodine, bromine, fluorine and the like, and among them, chlorine and fluorine are preferable, and fluorine is more preferable.
The acid anhydride represented by the above formula (8) or (9) may have a substituent other than halogen. Examples of the substituent include, but are not limited to, an alkyl group having 1 to 5 carbon atoms, an alkenyl group having 2 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, an aryl group having 6 to 12 carbon atoms, an amino group, a carboxyl group, a hydroxyl group, and a cyano group. For example, R 1 、R 2 Or R 3 At least 1 hydrogen atom of the saturated or unsaturated hydrocarbon ring is optionally substituted by an alkyl group having 1 to 3 carbon atoms.
Examples of the carboxylic acid anhydride include chain acid anhydrides such as acetic anhydride, propionic anhydride, butyric anhydride, crotonic anhydride and benzoic anhydride; anhydrides having a cyclic structure (cyclic anhydrides) such as succinic anhydride, glutaric anhydride, maleic anhydride, phthalic anhydride, 5, 6-dihydroxy-1, 4-dithienyl-2, 3-dicarboxylic anhydride, 5-norbornene-2, 3-dicarboxylic anhydride, 1, 2, 3, 6-tetrahydrophthalic anhydride, bicyclo [2.2.2] oct-5-ene-2, 3-dicarboxylic anhydride and the like.
Examples of the halide include Difluoroacetic anhydride (Difluoroacetic anhydride), 3H-perfluoropropionic anhydride (3H-perfluoropropionic anhydride), 3, 3, 3-Trifluoropropionic anhydride (3, 3, 3-Trifluoropropionic anhydride), pentafluoropropionic anhydride, 2, 3, 3, 4, 4-hexafluoropentanoic dianhydride, tetrafluorosuccinic anhydride, and trifluoroacetic anhydride. Further, an acid anhydride having another substituent such as 4-methylphthalic anhydride may be used in addition to the halide.
Examples of the sulfonic anhydride include chain sulfonic anhydrides such as methane sulfonic anhydride, ethane sulfonic anhydride, propane sulfonic anhydride, butane sulfonic anhydride, pentane sulfonic anhydride, hexane sulfonic anhydride, vinyl sulfonic anhydride, and benzene sulfonic anhydride; cyclic sulfonic anhydrides such as 1, 2-ethanedisulfonic anhydride, 1, 3-propanedisulfonic anhydride, 1, 4-butanedisulfonic anhydride and 1, 2-benzenedisulfonic anhydride; and halides thereof, and the like.
Examples of the anhydride of the carboxylic acid and the sulfonic acid include chain acid anhydrides such as acetic methane sulfonic anhydride, acetic ethane sulfonic anhydride, acetic propane sulfonic anhydride, propionic methane sulfonic anhydride, propionic ethane sulfonic anhydride, and propionic propane sulfonic anhydride; cyclic acid anhydrides such as 3-sulfopropionic anhydride, 2-methyl-3-sulfopropionic anhydride, 2-dimethyl-3-sulfopropionic anhydride, 2-ethyl-3-sulfopropionic anhydride, 2-diethyl-3-sulfopropionic anhydride and 2-sulfobenzoic anhydride; and halides thereof, and the like.
Among them, the acid anhydride is preferably a carboxylic anhydride having a structure represented by [ - (C ═ O) -O- (C ═ O) - ] in the molecule. Examples of the carboxylic anhydride include a chain carboxylic anhydride represented by the following formula (8-1) and a cyclic carboxylic anhydride represented by the following formula (9-1).
[ solution 9]
Figure BDA0002163473440000321
[ solution 10]
Figure BDA0002163473440000322
In the formulae (8-1) and (9-1), R 1 、R 2 And R 3 The groups shown are the same as those exemplified in the above formulae (8) and (9).
Examples of the preferable compounds of the acid anhydride include acetic anhydride, maleic anhydride, phthalic anhydride, propionic anhydride, succinic anhydride, benzoic anhydride, 5, 6-dihydroxy-1, 4-dithiine-2, 3-dicarboxylic anhydride, 5-norbornene-tetrahydrophthalic anhydride, bicyclo [2.2.2] oct-5-ene-2, 3-dicarboxylic anhydride, and the like; acid anhydrides having halogen or other substituent such as difluoroacetic anhydride, 3H-perfluoropropionic anhydride, trifluoropropionic anhydride, pentafluoropropionic anhydride, 2, 3, 3, 4, 4-hexafluoropentanedioic anhydride, tetrafluorosuccinic anhydride, trifluoroacetic anhydride, 4-methylphthalic anhydride, and the like.
The nonaqueous electrolytic solvent may include the following nonaqueous electrolytic solvents in addition to the above. The nonaqueous electrolytic solvent may contain, for example, γ -lactones such as γ -butyrolactone, cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, and the like. Further, these materials may contain a material in which a part of hydrogen atoms is substituted with fluorine atoms. In addition, aprotic organic solvents such as dimethylsulfoxide, 1, 3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propionitrile, nitromethane, ethylene glycol diethyl ether, trimethoxymethane, a dioxolane derivative, methyl sulfolane, 1, 3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidone, a propylene carbonate derivative, a tetrahydrofuran derivative, diethyl ether, 1, 3-propane sultone, anisole, and N-methylpyrrolidone may be contained. In addition, cyclic sulfonates may also be included. For example, the cyclic monosulfonate is preferably a compound represented by the following formula (10).
[ solution 11]
Figure BDA0002163473440000331
(in the above formula (10), R 101 And R 102 Each independently represents a hydrogen atom, a fluorine atom or an alkyl group having 1 to 4 carbon atoms. n is 0, 1, 2, 3 or 4. ).
Further, for example, the cyclic disulfonate ester is preferably a compound represented by the following formula (10-1).
[ solution 12]
Figure BDA0002163473440000341
(in the above formula (10-1), R 201 ～R 204 Each independently represents a hydrogen atom, a fluorine atom or an alkyl group having 1 to 4 carbon atoms. n is 0, 1, 2, 3 or 4. ).
Examples of the cyclic sulfonic acid ester include monosulfonic acid esters such as 1, 3-propanesultone, 1, 2-propanesultone, 1, 4-butanesultone, 1, 2-butanesultone, 1, 3-butanesultone, 2, 4-butanesultone, and 1, 3-pentanesulfontone; disulfonates such as methylene methane disulfonate and ethylene methane disulfonate. Among these, 1, 3-propane sultone, 1, 4-butane sultone, and methylene methane disulfonate are preferable from the viewpoints of film formation effect, acquisition easiness, and cost.
The content of the cyclic sulfonate in the electrolyte is preferably 0.01 to 10% by mass, more preferably 0.1 to 5% by mass. When the content of the cyclic sulfonic acid ester is 0.01 mass% or more, the coating film can be more effectively formed on the surface of the positive electrode, and decomposition of the electrolyte can be suppressed.
Examples of the supporting salt include LiPF 6 、LiAsF 6 、LiAlCl 4 、LiClO 4 、LiBF 4 、LiSbF 6 、LiCF 3 SO 3 、LiC 4 F 9 CO 3 、LiC(CF 3 SO 2 ) 2 、LiN(CF 3 SO 2 ) 2 、LiN(C 2 F 5 SO 2 ) 2 、LiB 10 Cl 10 And the like lithium salts. In addition, examples of the supporting salt include lithium lower aliphatic carboxylate, lithium borane chloride, lithium tetraphenylborate, LiBr, LiI, and,LiSCN, LiCl, etc. The supporting salt may be used singly or in combination of two or more.
Further, an ion-conductive polymer may be added to the nonaqueous electrolytic solvent. Examples of the ion conductive polymer include polyethers such as polyethylene oxide and polypropylene oxide; polyolefins such as polyethylene and polypropylene. Examples of the ion conductive polymer include polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyvinyl chloride, polyvinylidene chloride, polymethyl methacrylate, polymethyl acrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinyl pyrrolidone, polycarbonate, polyethylene terephthalate, polyhexamethylene adipamide, polycaprolactam, polyurethane, polyethyleneimine, polybutadiene, polystyrene, polyisoprene, and derivatives thereof. The ion-conductive polymer may be used singly or in combination of two or more. Further, a polymer containing various monomers constituting the above polymer may also be used.
The solid electrolyte is not particularly limited as long as it has the function of a solid electrolyte, and for example, Na- β -Al can be used 203 Or polyethylene oxide (PEO) as a polymer solid electrolyte, an oxide ion conductor called LISICON (1 hitium superior conductor), or a sulfide-based solid electrolyte (thio-LISICON, etc.).
(shape of Secondary Battery)
Examples of the shape of the secondary battery 10 according to the present embodiment include a cylindrical shape, a rectangular shape, a coin shape, a button shape, and a laminate shape. Examples of the exterior bodies 6 and 7 of the secondary battery 10 include stainless steel, iron, aluminum, titanium, an alloy thereof, and a plated product thereof. As the plating layer, for example, a nickel plating layer can be used.
Examples of the laminate resin film used in the laminate type include aluminum, aluminum alloy, and titanium foil. Examples of the material of the thermally fused portion of the metal laminated resin film include thermoplastic polymer materials such as polyethylene, polypropylene, and polyethylene terephthalate. The metal laminate resin layer and the metal foil layer are not limited to 1 layer, and may be 2 or more layers.
The outer packages 6 and 7 may be appropriately selected as long as they are stable against the electrolytic solution and have sufficient water vapor barrier properties. For example, in the case of a laminate type secondary battery, a laminate film of aluminum, polypropylene coated with silica, polyethylene, or the like can be used as the outer package. In particular, an aluminum laminated film is preferably used from the viewpoint of suppressing volume expansion.
Examples
The present invention is not limited to the embodiments described below, and can be implemented with appropriate modifications within a range not exceeding the gist of the embodiments.
(example 1)
As the negative electrode active material in this example, artificial graphite coated with a low crystalline carbon material was used. Artificial graphite coated with a low crystalline carbon material, a conductive aid as a spherical carbon material, and a binder for a negative electrode were mixed at a mass ratio of 97.7/0.3/2, and dispersed in N-methylpyrrolidone to prepare a slurry for a negative electrode. The slurry for a negative electrode was uniformly applied to a Cu current collector having a thickness of 10 μm. After drying, the negative electrode was produced by compression molding using a roll press.
As the nonaqueous electrolytic solvent, a solvent obtained by mixing Ethylene Carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 3/7 was used. Hereinafter, the solvent is also referred to simply as solvent EC/DEC. Make LiPF 6 The electrolyte solution was prepared by dissolving the compound in the nonaqueous electrolytic solvent at a concentration of 1 mol/L.
LiNiO to be used as a positive electrode active material 2 And Li 2 MnO 3 The solid solution of (4%) and polyvinylidene fluoride (4 mass%) as a binder for a positive electrode were mixed with carbon black as a conductive aid to prepare a positive electrode mixture. The positive electrode mixture was dispersed in N-methyl-2-pyrrolidone (NMP), thereby preparing a positive electrode slurry in which the positive electrode mixture was uniformly dispersed in a NMP solution of PVDF. The slurry for a positive electrode was uniformly applied to both surfaces of an aluminum positive electrode current collector having a thickness of 20 μm. Will be right atAfter the coating layer of the slurry for a positive electrode is dried, the coating layer is compression-molded by a roll press to form a positive electrode active material, thereby producing a positive electrode.
Fig. 3 shows the relationship between the first charge/discharge curves of the obtained negative electrode and positive electrode when they are combined. In this configuration, since the irreversible capacity of the positive electrode is larger than that of the negative electrode, "offset" is present in comparison with the target positional relationship shown in fig. 2, and a process for correcting the offset is required.
As one method, there is a method of consuming lithium of the positive electrode. For example, by impregnating the positive electrode with NO 2 BF 4 The solution reacts to chemically desorb lithium from the positive electrode, and the offset amount of lithium is removed from the positive electrode. The conditions for removing the desired amount are determined by evaluating the relationship between the removal amount and the solution concentration and the immersion time in advance. By combining the negative electrode and the positive electrode after the treatment, the discharge curves of the positive and negative electrodes show the relationship of fig. 4, and the relationship of fig. 2 can be maintained from the initial discharge.
As a separator for separating the positive electrode and the negative electrode, a microporous polypropylene film having a thickness of 25 μm was used.
The positive electrode, the negative electrode, and the separator prepared in the above manner were each processed into a predetermined shape, and a positive electrode having positive electrode active material layers on both surfaces, a negative electrode having negative electrode active material layers on both surfaces, and a separator were prepared. The plurality of positive electrodes and the plurality of negative electrodes are stacked with a separator interposed therebetween, respectively, to assemble the electrode element. The obtained electrode element was wrapped with an aluminum laminate film as an outer package, and an electrolytic solution was injected into the interior. Thereafter, for example, a lithium ion secondary battery can be manufactured by sealing under a reduced pressure atmosphere of 0.1 atm. A positive electrode tab is connected to the positive electrode current collector of the positive electrode, and a negative electrode tab is connected to the negative electrode current collector of the negative electrode, so that the positive electrode and the negative electrode can be electrically connected to each other via the positive electrode tab and the negative electrode tab from the outside of the outer package.
Since the positive and negative charge-discharge characteristics of the lithium ion secondary battery in the initial stage of use of the product are in the relationship shown in fig. 2, the positive electrode potential rises to terminate the charge, and the positive electrode potential falls to terminate the discharge. When lithium ions are lost due to decomposition reaction of the electrolyte solution and reaction with eluted electrode components during charge and discharge cycles, lithium in the negative electrode is released until the discharge of the positive electrode is completed, and thus the capacity hardly changes. At this time, the relationship of the positive and negative characteristics is: the charge-discharge curve of the positive electrode is shifted leftward with respect to the charge-discharge curve of the negative electrode. When this shift progresses, the amount of increase in the negative electrode potential during discharge gradually increases, and finally, the end of discharge is determined by the increase in the negative electrode potential. In this way, the capacity reduction is started with the displacement, but the displacement can be used until the lower limit of the capacity required for use as a battery is reached. In this way, the present embodiment can realize a long-life battery.
(example 2)
As a negative electrode material of this example, polyimide was mixed with the material of example 1. The slurry for a negative electrode was uniformly coated on a Cu current collector having a thickness of 10 μm. After drying, the resultant was heat-treated at 200 ℃ for 2 hours, and then compression-molded by a roll press to produce a negative electrode.
A nonaqueous electrolytic solvent and a positive electrode were produced in the same manner as in example 1. The positive electrode was not chemically subjected to lithium desorption treatment.
As the separator for separating the positive electrode and the negative electrode, a film obtained by laminating a microporous polyethylene having a thickness of 20 μm and an aramid was used.
The method for manufacturing the lithium ion secondary battery was the same as in example 1.
In this example, since the polyimide in the negative electrode has a property of irreversibly storing a part of lithium, the amount of lithium stored in the polyimide is inactivated from among lithium intercalated from the positive electrode to the negative electrode. By evaluating the relationship between the amount of polyimide and the amount of deactivated lithium in advance, a desired amount of lithium can be removed, and the relationship between the charge and discharge characteristics of the positive and negative electrodes in fig. 5 can be realized by adjusting the shift in fig. 3.
This embodiment can control the amount of lithium removed by the amount of material mixed, and therefore has the advantage of ease of manufacture.
Comparative example 1
Comparative example 1 is a lithium ion secondary battery having the same structure as in example 1, but the positive electrode is not chemically subjected to lithium desorption treatment.
At this time, the relationship between the charge and discharge curves of the positive electrode and the negative electrode is as shown in fig. 3. At this time, since the amount of lithium ions released from the positive electrode exceeds the negative electrode capacity, lithium is deposited on the negative electrode, and short-circuiting with the positive electrode may occur, causing smoke generation and ignition.
(example 3)
As the negative electrode active material of the present example, SiO coated with a low crystalline carbon material was used. SiO, a conductive aid as a spherical carbon material, a conductive aid as a flaky carbon material, and polyimide were mixed and dispersed in N-methylpyrrolidone to prepare a slurry for a negative electrode. The slurry for a negative electrode was uniformly applied to a stainless steel current collector having a thickness of 10 μm. After drying, the resultant was heat-treated at 240 ℃ for 1 hour, and compression-molded by a roll press to produce a negative electrode.
As the nonaqueous electrolytic solvent, the same solvent as in example 1 was used.
LiNi as a positive electrode active material 0.85 Co 0.10 Al 0.05 O 2 A positive electrode mixture was prepared by mixing polyvinylidene fluoride (4 mass%) as a binder for a positive electrode and carbon black as a conductive aid. By dispersing the positive electrode mixture in N-methyl-2-pyrrolidone, a positive electrode slurry in which a positive electrode active material and carbon black are uniformly dispersed in an NMP solution of PVDF is prepared. The slurry for a positive electrode was uniformly applied to both surfaces of an aluminum positive electrode current collector having a thickness of 20 μm. The coating layer of the slurry for a positive electrode was dried and then compression-molded by a roll press to form a positive electrode active material, thereby producing a positive electrode.
Fig. 6 shows the relationship between the initial charge/discharge curves when the obtained negative electrode and positive electrode are combined. In this configuration, since the irreversible capacity of the negative electrode is larger than that of the positive electrode, the negative electrode is "offset" from the target positional relationship shown in fig. 2, and a process for correcting the offset is required.
As one method, there is a method of adding lithium to the negative electrode in advance before assembly. For example, the following methods are used: a method of evaporating Li on the surface of the negative electrode and then diffusing it by heat treatment; and a method of immersing a negative electrode in an electrolyte solution in which a Li metal is disposed as a positive electrode, and applying a potential between the positive electrode and the negative electrode to cause the negative electrode to store Li. The control of the amount of lithium added was performed by previously evaluating the relationship between the amount of lithium and the amount of current and the immersion time. By combining the negative electrode and the positive electrode after the treatment, the discharge curves of the positive and negative electrodes show the relationship of fig. 7, and can be kept consistent with the desired relationship of fig. 2 from the time of initial discharge.
As a separator for separating the positive electrode from the negative electrode, a microporous polyethylene film having a thickness of 25 μm was used.
The lithium ion secondary battery was produced in the same manner as in example 1.
In the present embodiment, the irreversible capacity of the negative electrode is large, and when the relationship between the positive and negative electrodes is as shown in fig. 6, the relationship shown in fig. 2 can be corrected.
(example 4)
The negative electrode of this example was the same as example 3.
LiNi as a positive electrode active material 0.5 Mn 1.5 O 4 A positive electrode mixture was prepared by mixing polyvinylidene fluoride (4 mass%) as a binder for a positive electrode and carbon black as a conductive aid.
When the obtained negative electrode and positive electrode were used as they were, the charge/discharge curves of the positive and negative electrodes showed the relationship of fig. 6. As a method for correcting the offset, there is a method of adding lithium to the positive electrode before assembly. For example, a positive electrode is placed in an electrolyte solution in which lithium metal is disposed as a negative electrode, and a voltage is applied between the positive and negative electrodes to cause discharge. The positive electrode material of the present embodiment has a capacity in the discharge region, and thus lithium is stored.
A nonaqueous electrolytic solvent was prepared in the same manner as in example 1.
As the separator for separating the positive and negative electrodes, a film in which microporous polyethylene having a thickness of 20 μm and aramid were laminated was used.
The method for manufacturing the lithium ion secondary battery was the same as in example 1.
In the present embodiment, the battery is overdischarged at the positive electrode, and the offset in fig. 6 is adjusted to the relationship in fig. 8, whereby the relationship in fig. 2 can be maintained from the time of initial discharge.
Comparative example 2
Comparative example 2 is a lithium ion secondary battery having the same configuration as in example 3, but with no lithium addition to the negative electrode.
At this time, the relationship between the charge and discharge curves of the positive electrode and the negative electrode is as shown in fig. 6. At this time, if lithium is mixed into a substance generated by decomposition of the electrolyte during charge and discharge cycles and disappears, lithium inserted into the positive electrode during discharge decreases, and thus the capacity decreases.
The present invention has been described above with reference to the embodiments and examples, but the present invention is not limited to the embodiments and examples. Various modifications can be made in the structure and details of the present invention as will be apparent to those skilled in the art within the scope of the present invention.
The present application claims priority based on japanese application special application No. 2017-031348 filed on 22/2/2017, the entire disclosure of which is incorporated herein by reference.

Claims (12)

1. A secondary battery at least comprises:
a positive electrode;
a negative electrode;
a separation layer spatially separating the positive electrode and the negative electrode; and
an ion conductor held between the positive electrode and the negative electrode and having a function of conducting ions between the positive electrode and the negative electrode,
the secondary battery has the following characteristics in the initial stage of use:
a characteristic in which a rate of decrease in the potential of the positive electrode immediately before completion of full discharge is greater than a rate of increase in the potential of the negative electrode immediately before completion of full discharge; and
a characteristic that a rate of increase in the potential of the positive electrode immediately before completion of full charge is larger than a rate of decrease in the potential of the negative electrode immediately before completion of full charge,
the secondary battery is continuously used until the potential decrease rate of the positive electrode immediately before the end of full discharge is smaller than the potential increase rate of the negative electrode immediately before the end of full discharge,
at the initial stage of use of the secondary battery,
an absolute value Δ V of a potential change amount per 10mAh/g of the positive electrode immediately before completion of full discharge when discharging at a constant current of 1/20C 2 An absolute value DeltaV of a potential change amount per 10mAh/g of the negative electrode immediately before completion of full discharge 1 Ratio of delta V 2 /ΔV 1 Satisfies Δ V 2 /ΔV 1 A relationship of > 1.
2. A secondary battery at least comprises:
a positive electrode;
a negative electrode;
a separation layer spatially separating the positive electrode and the negative electrode; and
an ion conductor held between the positive electrode and the negative electrode and having a function of conducting ions between the positive electrode and the negative electrode,
the secondary battery has the following characteristics in the initial stage of use:
a characteristic in which a rate of decrease in the potential of the positive electrode immediately before completion of full discharge is greater than a rate of increase in the potential of the negative electrode immediately before completion of full discharge; and
a characteristic that a rate of increase in the potential of the positive electrode immediately before completion of full charge is larger than a rate of decrease in the potential of the negative electrode immediately before completion of full charge,
the secondary battery is continuously used until the potential decrease rate of the positive electrode immediately before the end of full discharge is smaller than the potential increase rate of the negative electrode immediately before the end of full discharge,
at the initial stage of use of the secondary battery,
an absolute value Δ V of a potential change amount per 10mAh/g of the positive electrode immediately before completion of full charge when charging is performed at a constant current of 1/20C 4 An absolute value Δ V of a potential change amount per 10mAh/g of the negative electrode immediately before completion of full charge 3 Ratio of delta V 4 /ΔV 3 Satisfies Δ V 4 /ΔV 3 A relationship of > 1.
3. A secondary battery at least comprises:
a positive electrode;
a negative electrode;
a separation layer spatially separating the positive electrode and the negative electrode; and
an ion conductor held between the positive electrode and the negative electrode and having a function of conducting ions between the positive electrode and the negative electrode,
the secondary battery has the following characteristics in the initial stage of use:
a characteristic in which a rate of decrease in the potential of the positive electrode immediately before completion of full discharge is greater than a rate of increase in the potential of the negative electrode immediately before completion of full discharge; and
a characteristic that a rate of increase in the potential of the positive electrode immediately before completion of full charge is larger than a rate of decrease in the potential of the negative electrode immediately before completion of full charge,
the secondary battery is continuously used until the potential decrease rate of the positive electrode immediately before the end of full discharge is smaller than the potential increase rate of the negative electrode immediately before the end of full discharge,
the secondary battery is continuously used until the absolute value DeltaV of the potential change amount per 10mAh/g of the positive electrode immediately before the full discharge is finished when the discharge is performed at a constant current of 1/20C 2 An absolute value DeltaV of a potential change amount per 10mAh/g of the negative electrode immediately before completion of full discharge 1 Ratio of delta V 2 /ΔV 1 Satisfies Δ V 2 /ΔV 1 The state of the relationship < 1.
4. The secondary battery according to any one of claims 1 to 3,
at the beginning of use of the secondary battery, at the end of full discharge, the negative electrode has excess conductive ions.
5. The secondary battery according to claim 4,
when the amount of conductive ions contributing to charge and discharge decreases during the use period of the secondary battery, the decreased conductive ions are compensated for by the excess conductive ions in the negative electrode.
6. The secondary battery according to any one of claims 1 to 3, which is a lithium ion secondary battery.
7. A method for using a secondary battery, the method comprising at least:
a positive electrode;
a negative electrode;
a separation layer spatially separating the positive electrode and the negative electrode; and
an ion conductor held between the positive electrode and the negative electrode and having a function of conducting ions between the positive electrode and the negative electrode,
at the initial stage of use of the secondary battery,
under the condition that the rate of decrease in the potential of the positive electrode immediately before the end of full discharge is greater than the rate of increase in the potential of the negative electrode immediately before the end of full discharge, and under the condition that the rate of increase in the potential of the positive electrode immediately before the end of full charge is greater than the rate of decrease in the potential of the negative electrode immediately before the end of full charge,
the secondary battery is continuously used until the potential decrease rate of the positive electrode immediately before the end of full discharge is smaller than the potential increase rate of the negative electrode immediately before the end of full discharge,
at the initial stage of use of the secondary battery,
an absolute value Δ V of a potential change amount per 10mAh/g of the positive electrode immediately before completion of full discharge when discharging at a constant current of 1/20C 2 An absolute value DeltaV of a potential change amount per 10mAh/g of the negative electrode immediately before completion of full discharge 1 Ratio of delta V 2 /ΔV 1 Satisfies Δ V 2 /ΔV 1 Condition of relation > 1The preparation is used.
8. A method for using a secondary battery, the method comprising at least:
a positive electrode;
a negative electrode;
a separation layer spatially separating the positive electrode and the negative electrode; and
an ion conductor held between the positive electrode and the negative electrode and having a function of conducting ions between the positive electrode and the negative electrode,
at the initial stage of use of the secondary battery,
under the condition that the rate of decrease in the potential of the positive electrode immediately before the end of full discharge is greater than the rate of increase in the potential of the negative electrode immediately before the end of full discharge, and under the condition that the rate of increase in the potential of the positive electrode immediately before the end of full charge is greater than the rate of decrease in the potential of the negative electrode immediately before the end of full charge,
the secondary battery is continuously used until the potential decrease rate of the positive electrode immediately before the end of full discharge is smaller than the potential increase rate of the negative electrode immediately before the end of full discharge,
at the initial stage of use of the secondary battery,
a change in potential Δ V per 10mAh/g of the positive electrode immediately before completion of full charge when charging is performed at a constant current of 1/20C 4 Is relative to the potential change amount DeltaV per 10mAh/g of the negative electrode immediately before the end of full charge 3 Is Δ V of 4 /ΔV 3 Satisfies Δ V 4 /ΔV 3 The relationship > 1.
9. A method for using a secondary battery, the method comprising at least:
a positive electrode;
a negative electrode;
a separation layer spatially separating the positive electrode and the negative electrode; and
an ion conductor held between the positive electrode and the negative electrode and having a function of conducting ions between the positive electrode and the negative electrode,
at the initial stage of use of the secondary battery,
under the condition that the rate of decrease in the potential of the positive electrode immediately before the end of full discharge is greater than the rate of increase in the potential of the negative electrode immediately before the end of full discharge, and under the condition that the rate of increase in the potential of the positive electrode immediately before the end of full charge is greater than the rate of decrease in the potential of the negative electrode immediately before the end of full charge,
the secondary battery is continuously used until the potential decrease rate of the positive electrode immediately before the end of full discharge is smaller than the potential increase rate of the negative electrode immediately before the end of full discharge,
the secondary battery is continuously used until the absolute value DeltaV of the potential change amount per 10mAh/g of the positive electrode immediately before the full discharge is finished when the discharge is performed at a constant current of 1/20C 2 An absolute value DeltaV of a potential change amount per 10mAh/g of the negative electrode immediately before completion of full discharge 1 Ratio of delta V 2 /ΔV 1 Satisfies Δ V 2 /ΔV 1 The state of the relationship < 1.
10. The method for using a secondary battery according to any one of claims 7 to 9,
at the beginning of use of the secondary battery, at the end of full discharge, the negative electrode has excess conductive ions.
11. The method for using a secondary battery according to claim 10,
when the amount of conductive ions contributing to charge and discharge decreases during the use period of the secondary battery, the decreased conductive ions are filled with the remaining conductive ions in the negative electrode.
12. The method for using a secondary battery according to any one of claims 7 to 9,
the secondary battery is a lithium ion secondary battery.
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